Introduce has_header_image() to check whether a header image is set.

props GunGeekATX, voldemortensen.
fixes #16268.

git-svn-id: https://develop.svn.wordpress.org/trunk@31224 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -1034,6 +1034,17 @@ function display_header_text() {
 	return 'blank' != $text_color;
 }
 
+/**
+ * Check whether a header image is set or not.
+ *
+ * @since 4.2.0
+ *
+ * @return bool
+ */
+function has_header_image() {
+	return (bool) get_header_image();
+}
+
 /**
  * Retrieve header image for custom header.
  *