fix(share_plus): Add missing call to result for shareUri on iOS

[miquelbeltran]

Description

• The code was never completing when being awaited.
• This is due to the missing call to result(nil).
• Tested on iOS simulator using example project.

Related Issues

• Fix [Bug]: Share Plus calling Share.shareUri return a never complete future #2609

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[miquelbeltran]

withResult may always be false in shareUri, so I am tempted to just always call to result(nil).

However, since this value comes from the Dart side, we cannot warranty that in native code. That's why I added the if-condition here.