Add checkers and tests for empty locale in PNG file

match_locale() will return false for empty locale string in the PNG
file. Also add a manual test to validate if a PNG file is qualified to
use under recovery.

Bug: 34054052
Test: recovery_manual_test catches invalid PNG files successfully & Locale_test passed

Change-Id: Id7e2136e1d8abf20da15825aa7901effbced8b03
diff --git a/minui/resources.cpp b/minui/resources.cpp
index e6909f2..726c627 100644
--- a/minui/resources.cpp
+++ b/minui/resources.cpp
@@ -374,7 +374,9 @@
 // This function tests if a locale string stored in PNG (prefix) matches
 // the locale string provided by the system (locale).
 bool matches_locale(const char* prefix, const char* locale) {
-    if (locale == NULL) return false;
+    if (locale == nullptr) {
+        return false;
+    }
 
     // Return true if the whole string of prefix matches the top part of
     // locale. For instance, prefix == "en" matches locale == "en_US";