Improve handling of Scan permissions

Previously they could be rejected with an empty grant results
array, which crashed here. With this, that should be considered
as a clean cancellation instead.

Author: pimterry

app/src/main/java/tech/httptoolkit/android/ScanActivity.kt

@@ -57,7 +57,7 @@ class ScanActivity : AppCompatActivity(), ZXingScannerView.ResultHandler {
         grantResults: IntArray
     ) {
         super.onRequestPermissionsResult(requestCode, permissions, grantResults)
-        if (grantResults[0] == PERMISSION_GRANTED) {
+        if (grantResults.isNotEmpty() && grantResults[0] == PERMISSION_GRANTED) {
             Log.i(TAG, "Camera permissions granted")
         } else {
             Log.i(TAG, "Camera permissions rejected")