fix(share_plus): Ensure subject is not null before calling putExtra(Intent.EXTRA_SUBJECT, subject)

[jmsheff]

Description

My friend who would share links via share_plus through Verizon's Message+ app, faced an issue where there would always be an additional null message alongside the shared link:

After doing a lot of digging and trying different things to try to fix this, I noticed that Intent.EXTRA_SUBJECT is always set to null when a subject is not specified. I suspected that this was the issue, given it was only happening for this one messaging platform, which may not filter out a null subject like most others do. Sure enough, after using my fork in place of the official share_plus package and having my friend send me a link again, this issue was fixed. I suspect that there may be other platforms affected by this as well.

Related Issues

N/A

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.
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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.

[vbuberen]

Thanks for your investigation and contribution. Looks good to me.

Will release next week.