Remove deprecations in PHP 8.1

Hi! We are seeing several deprecation warnings when running a WordPress site under PHP 8.1 with your plugin.

Here I’m attaching a patch to fix them.

diff --git a/src/css-optimize.php b/src/css-optimize.php
index ece4b43281..2a7d6dcc14 100644
--- a/src/css-optimize.php
+++ b/src/css-optimize.php
@@ -140,7 +140,7 @@ if ( ! class_exists( 'Stackable_CSS_Optimize' ) ) {
 		 */
 		public function parse_blocks( $blocks, &$style_arr ) {
 			foreach ( $blocks as $block ) {
-				if ( stripos( $block['blockName'], 'stackable/' ) !== false ) {
+				if ( stripos( $block['blockName'] ?? '', 'stackable/' ) !== false ) {
 					$this->parse_block_style( $block, $style_arr );
 				}
 
@@ -262,8 +262,8 @@ if ( ! class_exists( 'Stackable_CSS_Optimize' ) ) {
 			}
 
 			// Only do this to our blocks.
-			if ( ! empty( $block ) && is_array( $block ) && stripos( $block['blockName'], 'stackable/' ) === 0 ) {
-				if ( stripos( $block_content, '<style' ) !== false ) {
+			if ( ! empty( $block ) && is_array( $block ) && stripos( $block['blockName'] ?? '', 'stackable/' ) === 0 ) {
+				if ( stripos( $block_content ?? '', '<style' ) !== false ) {
 
 					// We need the unique id for tracking.
 					if ( is_array( $block['attrs'] ) && array_key_exists( 'uniqueId', $block['attrs'] ) ) {
diff --git a/src/fonts.php b/src/fonts.php
index 648e7e8ae9..6d7f93ef2e 100644
--- a/src/fonts.php
+++ b/src/fonts.php
@@ -39,7 +39,7 @@ if ( ! class_exists( 'Stackable_Google_Fonts' ) ) {
 		}
 
 		public function is_stackable_block( $block_name ) {
-			return strpos( $block_name, 'ugb/' ) === 0 || strpos( $block_name, 'stackable/' ) === 0;
+			return strpos( $block_name ?? '', 'ugb/' ) === 0 || strpos( $block_name ?? '', 'stackable/' ) === 0;
 		}
 
 		public static function register_font( $font_name ) {
diff --git a/src/global-settings.php b/src/global-settings.php
index a82b691db5..223e16de82 100644
--- a/src/global-settings.php
+++ b/src/global-settings.php
@@ -765,7 +765,7 @@ if ( ! class_exists( 'Stackable_Global_Settings' ) ) {
 			}
 
 			// Only do this for native blocks.
-			if ( stripos( $block['blockName'], 'core/' ) !== 0 ) {
+			if ( stripos( $block['blockName'] ?? '', 'core/' ) !== 0 ) {
 				return $block_content;
 			}
 
diff --git a/src/init.php b/src/init.php
index 3708e48155..5fe16281c9 100644
--- a/src/init.php
+++ b/src/init.php
@@ -192,7 +192,7 @@ if ( ! class_exists( 'Stackable_Init' ) ) {
 			// frontend.
 			if ( ! $this->is_main_script_loaded && ! is_admin() ) {
 				if (
-					stripos( $block['blockName'], 'stackable/' ) === 0 ||
+					stripos( $block['blockName'] ?? '', 'stackable/' ) === 0 ||
 					stripos( $block_content, '<!-- wp:stackable/' ) !==  false ||
 					stripos( $block_content, 'stk-highlight' ) !==  false
 				) {
@@ -203,8 +203,8 @@ if ( ! class_exists( 'Stackable_Init' ) ) {
 
 			// Load our individual block script if they're used in the page.
 			$stackable_block = '';
-			if ( stripos( $block['blockName'], 'stackable/' ) === 0 ) {
-				if ( preg_match( '#stackable/([\w\d-]+)#', $block['blockName'], $matches ) ) {
+			if ( stripos( $block['blockName'] ?? '', 'stackable/' ) === 0 ) {
+				if ( preg_match( '#stackable/([\w\d-]+)#', $block['blockName'] ?? '', $matches ) ) {
 					$stackable_block = $matches[1];
 				}
 			} else if ( stripos( $block_content, '<!-- wp:stackable/' ) !==  false ) {
diff --git a/src/welcome/index.php b/src/welcome/index.php
index 2435860121..4e4d3eed40 100644
--- a/src/welcome/index.php
+++ b/src/welcome/index.php
@@ -36,7 +36,7 @@ if ( ! class_exists( 'Stackable_Welcome_Screen' ) ) {
 
 			// Our getting started page.
 			add_submenu_page(
-				isset( $_GET['page'] ) && $_GET['page'] === 'stackable-getting-started' ? 'options-general.php' : null, // Parent slug. Only show when in the page.
+				isset( $_GET['page'] ) && $_GET['page'] === 'stackable-getting-started' ? 'options-general.php' : '', // Parent slug. Only show when in the page.
 				__( 'Get Started', STACKABLE_I18N ), // Page title.
 				'<span class="fs-submenu-item fs-sub"></span>' . __( 'Get Started', STACKABLE_I18N ), // Menu title.
 				'manage_options', // Capability.
diff --git a/src/welcome/wizard.php b/src/welcome/wizard.php
index fe53638e4e..b30a5608fc 100644
--- a/src/welcome/wizard.php
+++ b/src/welcome/wizard.php
@@ -20,7 +20,7 @@ if ( ! class_exists( 'Stackable_Onboarding_Wizard' ) ) {
         public function add_dashboard_page() {
 			// Our wizard page.
 			add_submenu_page(
-				isset( $_GET['page'] ) && $_GET['page'] === 'stackable-settings-wizard' ? 'options-general.php' : null, // Parent slug. Only show when in the page.
+				isset( $_GET['page'] ) && $_GET['page'] === 'stackable-settings-wizard' ? 'options-general.php' : '', // Parent slug. Only show when in the page.
 				__( 'Wizard', STACKABLE_I18N ), // Page title.
 				'<span class="fs-submenu-item fs-sub"></span>' . __( 'Wizard', STACKABLE_I18N ), // Menu title.
 				'manage_options', // Capability.