[share] Resolve NPE on iOS

[ened]

Description

Using version 0.6.2+2 & the following code:

Share.share('https://www.flutter.dev');

will cause a hard crash on iOS if the user selects the Mail option in the share sheet.
This is due to a NSNull parameter being passed down to the plugin which, when assigned to the UIActivityViewController, causes the crash.

This PR also initialises a variable to reduce a compiler warning.

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[amirh]

Thanks, I believe this was just fixed in #2131

[ened]

OK.