Editor: Fix post editor layout when Post Content has no attributes.

Changes output of `wp_get_post_content_block_attributes` to return null if Post Content block doesn’t exist or empty array if it has no attributes.

Props flixos90, mukesh27.
Fixes #59358.


git-svn-id: https://develop.svn.wordpress.org/trunk@56629 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/themedir1/block-theme-post-content-default/style.css

@@ -0,0 +1,7 @@
+/*
+Theme Name: Block Theme Post Content Default
+Theme URI: https://wordpress.org/
+Description: For testing purposes only.
+Version: 1.0.0
+Text Domain: block-theme-post-content-default
+*/

tests/phpunit/data/themedir1/block-theme-post-content-default/templates/index.html

@@ -0,0 +1,3 @@
+<!-- wp:paragraph -->
+<p>Index Template</p>
+<!-- /wp:paragraph -->

tests/phpunit/data/themedir1/block-theme-post-content-default/templates/single.html

@@ -0,0 +1,2 @@
+<!-- wp:post-title {"level":1,"style":{"spacing":{"margin":{"bottom":"var:preset|spacing|40"}}}} /-->
+<!-- wp:post-content /-->

tests/phpunit/data/themedir1/block-theme-post-content-default/theme.json

@@ -0,0 +1,104 @@
+{
+	"version": 1,
+	"title": "Block theme",
+	"settings": {
+		"color": {
+			"palette": [
+				{
+					"slug": "light",
+					"name": "Light",
+					"color": "#f5f7f9"
+				},
+				{
+					"slug": "dark",
+					"name": "Dark",
+					"color": "#000"
+				}
+			],
+			"gradients": [
+				{
+					"name": "Custom gradient",
+					"gradient": "linear-gradient(135deg,rgba(0,0,0) 0%,rgb(0,0,0) 100%)",
+					"slug": "custom-gradient"
+				}
+			],
+			"duotone": [
+				{
+					"colors": [ "#333333", "#aaaaaa" ],
+					"slug": "custom-duotone",
+					"name": "Custom Duotone"
+				}
+			],
+			"custom": false,
+			"customGradient": false
+		},
+		"typography": {
+			"fontSizes": [
+				{
+					"name": "Custom",
+					"slug": "custom",
+					"size": "100px"
+				}
+			],
+			"customFontSize": false,
+			"customLineHeight": true
+		},
+		"spacing": {
+			"units": ["rem"],
+			"customPadding": true,
+			"blockGap": true
+		},
+		"blocks": {
+			"core/paragraph": {
+				"color": {
+					"palette": [
+						{
+							"slug": "light",
+							"name": "Light",
+							"color": "#f5f7f9"
+						}
+					]
+				}
+			}
+		}
+	},
+	"styles" : {
+		"blocks" :{
+			"core/post-featured-image": {
+				"shadow": "10px 10px 5px 0px rgba(0,0,0,0.66)",
+				"filter": {
+					"duotone": "var(--wp--preset--duotone--custom-duotone)"
+				}
+			}
+		},
+		"elements": {
+			"button": {
+				"shadow": "10px 10px 5px 0px rgba(0,0,0,0.66)"
+			},
+			"link": {
+				"typography": {
+					"textDecoration": "none"
+				},
+				"border": {
+					"bottom": {
+						"width": "2px",
+						"color": "currentColor",
+						"style": "solid"
+					}
+				},
+				":hover": {
+					"typography": {
+						"textDecoration": "none"
+					},
+					"border": {
+						"bottom": {
+							"width": "2px",
+							"color": "#000",
+							"style": "dotted"
+						}
+					}
+				}
+			}
+		}
+	}
+}