Editor: Avoid unnecessary array_merge in WP_Style_Engine::parse_block_styles().

This adds an `! empty()` check for classnames and declarations to avoid calling array_merge() with an empty value.

Props mukesh27, ramonopoly, aaronrobertshaw.
Fixes #62317.


git-svn-id: https://develop.svn.wordpress.org/trunk@59442 602fd350-edb4-49c9-b593-d223f7449a82

Author: joemcgill

src/wp-includes/style-engine/class-wp-style-engine.php

@@ -454,8 +454,15 @@ public static function parse_block_styles( $block_styles, $options ) {
 					continue;
 				}
 
-				$parsed_styles['classnames']   = array_merge( $parsed_styles['classnames'], static::get_classnames( $style_value, $style_definition ) );
-				$parsed_styles['declarations'] = array_merge( $parsed_styles['declarations'], static::get_css_declarations( $style_value, $style_definition, $options ) );
+				$classnames = static::get_classnames( $style_value, $style_definition );
+				if ( ! empty( $classnames ) ) {
+					$parsed_styles['classnames'] = array_merge( $parsed_styles['classnames'], $classnames );
+				}
+
+				$css_declarations = static::get_css_declarations( $style_value, $style_definition, $options );
+				if ( ! empty( $css_declarations ) ) {
+					$parsed_styles['declarations'] = array_merge( $parsed_styles['declarations'], $css_declarations );
+				}
 			}
 		}