RefreshLinksJob: Check hastext before comparing HTML

Why:
- When comparing the newly generated HTML to the cached HTML, there
  might be cases when the new ParserOutput doesn't contain HTML.

What:
- If hasText() returns false, don't compare HTML and use the "unknown"
  value for the html_changed stats label.

Bug: T388406
Change-Id: Ibc3e79e79a6421d4780739104a949bac50a5b01f
(cherry picked from commit a275e02771bc2ed4243804d5294188f54e47f9fc)

includes/jobqueue/jobs/RefreshLinksJob.php

@@ -458,8 +458,9 @@ class RefreshLinksJob extends Job {
 		// Collect stats on parses that don't actually change the page content.
 		// In that case, we could abort here, and perhaps we could also avoid
 		// triggering CDN purges (T369898).
-		if ( !$cachedOutput ) {
-			// There was no cached output
+		if ( !$cachedOutput || !$output->hasText() ) {
+			// There was no cached output, or no HTML was generated because
+			// shouldGenerateHTMLOnEdit returned false.
 			$htmlChanged = 'unknown';
 		} elseif ( $cachedOutput->getRawText() === $output->getRawText() ) {
 			// We have cached output, but we couldn't be sure that it was still good.