tests: utils: Allow NULL strings to be equal to empty file in virTestCompareToFile

Treat an NULL string equivalent to an empty string in
virTestCompareToFile so that callers don't need to add additional logic
in case when a test produces no output.

tests/testutils.c

@@ -777,6 +777,8 @@ int virTestDifferenceBin(FILE *stream,
 /*
  * @param strcontent: String input content
  * @param filename: File to compare strcontent against
+ *
+ * If @strcontent is NULL, it's treated as an empty string.
  */
 int
 virTestCompareToFile(const char *strcontent,
@@ -787,6 +789,9 @@ virTestCompareToFile(const char *strcontent,
     char *fixedcontent = NULL;
     const char *cmpcontent = strcontent;
 
+    if (!cmpcontent)
+        cmpcontent = "";
+
     if (virTestLoadFile(filename, &filecontent) < 0 && !virTestGetRegenerate())
         goto failure;