Avoid PHP notices when checking termmeta capabilities against a non-existent term.

Previously, checks like `current_user_can( 'edit_term_meta', $term_id )`
returned the proper value, but generated a PHP notice due to the fact
that `get_term( $term_id )` could, in certain instances, return
`WP_Error` objects.

Props caercam.
Fixes #40891.

git-svn-id: https://develop.svn.wordpress.org/trunk@40999 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/capabilities.php

@@ -280,7 +280,7 @@ function map_meta_cap( $cap, $user_id ) {
 
 			case 'term':
 				$term = get_term( $object_id );
-				if ( ! $term ) {
+				if ( ! $term instanceof WP_Term ) {
 					break;
 				}
 

tests/phpunit/tests/user/capabilities.php

@@ -1353,6 +1353,27 @@ class Tests_User_Capabilities extends WP_UnitTestCase {
 		}
 	}
 
+	/**
+	 * @ticket 40891
+	 */
+	public function test_taxonomy_meta_capabilities_with_non_existent_terms() {
+		$caps = array(
+			'add_term_meta',
+			'delete_term_meta',
+			'edit_term_meta',
+		);
+
+		$taxonomy = 'wptests_tax';
+		register_taxonomy( $taxonomy, 'post' );
+
+		$editor = self::$users['editor'];
+
+		foreach ( $caps as $cap ) {
+			// `null` represents a non-existent term ID.
+			$this->assertFalse( user_can( $editor->ID, $cap, null ) );
+		}
+	}
+
 	/**
 	 * @ticket 21786
 	 */