If there is no module name, module should be null

Author: supervacuus

sentry-android-core/src/main/java/io/sentry/android/core/internal/tombstone/TombstoneParser.java

@@ -227,7 +227,7 @@ private static String normalizeFunctionName(String fqFunctionName) {
     if (normalized.contains(".")) {
       return normalized.substring(0, normalized.lastIndexOf("."));
     } else {
-      return "";
+      return null;
     }
   }