Fix NPE when listing files in screenshot folder

[rafakob]

listFiles() may return null which leads to crash:

Exception in thread "main" java.lang.IllegalArgumentException: Parameter specified as non-null is null: method kotlin.collections.ArraysKt___ArraysKt.toList, parameter $receiver
        at kotlin.collections.ArraysKt___ArraysKt.toList(_Arrays.kt)
        at com.gojuno.composer.TestRunKt$pullTestFiles$3.call(TestRun.kt:177)
        at com.gojuno.composer.TestRunKt$pullTestFiles$3.call(TestRun.kt)

[plastiv]

How to reproduce this NPE?

it.listFiles() can return null only when it is file instead of folder. Who is creating it as a file?

Composer code doesn't seem to be touching it, creating parent testClassName folder only. So it looks like file was downloaded by adb pull. Which could happen only when not complying to spoon screenshot contract

storage/sdcard/app_spoon-screenshots/package.testClassName/testMethodName/screenshotName.png`

In this case, I think proposed solution to silently ignoring NPE is incorrect. Because storage/sdcard/app_spoon-screenshots/package.testClassName/testMethodName screenshot is not getting merged into html report. Instead, I would prefer to have an assertion message to avoid debugging why.