[Android] Check if keyStore path is empty

In `godot-build-scripts`, the default `config.sh` sets `GODOT_ANDROID_SIGN_KEYSTORE` to an empty string but we were only checking if it's null.

platform/android/java/editor/build.gradle

@@ -36,7 +36,7 @@ ext {
     // Return the keystore file used for signing the release build.
     getGodotKeystoreFile = { ->
         def keyStore = System.getenv("GODOT_ANDROID_SIGN_KEYSTORE")
-        if (keyStore == null) {
+        if (keyStore == null || keyStore.isEmpty()) {
             return null
         }
         return file(keyStore)