BLI_string_ref: avoid passing null to strncmp (quiet ASAN warning)

The existing logic to avoid passing null only worked when both
strings were null.

source/blender/blenlib/BLI_string_ref.hh

@@ -608,9 +608,14 @@ constexpr bool operator==(StringRef a, StringRef b)
     return false;
   }
   if (a.data() == b.data()) {
-    /* This also avoids passing null to the call below, which would results in an ASAN warning. */
+    /* This also avoids passing null to the call below when both are null,
+     * which would results in an ASAN warning. */
     return true;
   }
+  if (!a.data() || !b.data()) {
+    /* Account for a single value being null, resulting in an ASAN warning. */
+    return false;
+  }
   return STREQLEN(a.data(), b.data(), size_t(a.size()));
 }