Don't parse empty 'tax_input' keys in edit_post().

This fixes a bug introduced in [31359] where saving a tax_input that contained
only whitespace would result in a random tag being erroneously added to the
post.

See #30615.

git-svn-id: https://develop.svn.wordpress.org/trunk@31392 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/admin/includesPost.php

@@ -175,6 +175,36 @@ class Tests_Admin_includesPost extends WP_UnitTestCase {
 		$this->assertContains( 'baz', wp_list_pluck( $found, 'name' ) );
 	}
 
+	/**
+	 * @ticket 30615
+	 */
+	public function test_edit_post_should_not_create_terms_for_an_empty_tag_input_field() {
+		$u = $this->factory->user->create( array( 'role' => 'editor' ) );
+		wp_set_current_user( $u );
+
+		register_taxonomy( 'wptests_tax', array( 'post' ) );
+		$t1 = $this->factory->term->create( array(
+			'taxonomy' => 'wptests_tax',
+			'name' => 'foo',
+			'slug' => 'bar',
+		) );
+
+		$p = $this->factory->post->create();
+
+		$post_data = array(
+			'post_ID' => $p,
+			'tax_input' => array(
+				'wptests_tax' => ' ',
+			),
+		);
+
+		edit_post( $post_data );
+
+		$found = wp_get_post_terms( $p, 'wptests_tax' );
+
+		$this->assertEmpty( $found );
+	}
+
 	/**
 	 * @ticket 27792
 	 */