Fix crash on nil/invalid URL (iOS)

[AlexV525]

Connection with issue(s)

No unsolved issue, but might be similar to #74 and #174 .

Testing and Review Notes

Pass a url include whitespace (e.g. 'https://www.google.com '), app will crash immediately.
By adding this, there're still many codes using ! for force parsing, it require further modification.

To Do

Nothing needs to be handled.

[arnaudelub]

If you want, as the maintainer is out for quite a long time now, we are maintaining this fork:
https://github.com/arnaudelub/flutter_polywebview
package also available on pub.dev:
https://pub.dev/packages/flutter_polywebview

feel free to add you PR on this fork if you want.