fix: gradle export for APKs without strings.xml

[nitram84]

When I try to export this APK (https://python-helper.apk.gold/) as gradle project, I'm getting the same exception like in #2044 , #1920 and #1918

java.lang.IllegalStateException
	at java.base/java.util.Optional.orElseThrow(Optional.java:403)
	at jadx.core.export.ExportGradleTask.lambda$run$3(ExportGradleTask.java:58)
	at java.base/java.util.Optional.orElseGet(Optional.java:364)
	at jadx.core.export.ExportGradleTask.run(ExportGradleTask.java:55)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:833)

The gradle export always expects a strings.xml, but this assumption isn't true, so my proposition is to postpone this check. I'm sure this PR fixes will also fix #1920.

[skylot]

@nitram84 thanks!