Do not raise if source_code_uri is missing in the gemspec

[kennyadsl]

Summary

That metadata is useful to automatically generate the CHANGELOG based on GitHub activity, using Oktokit and github_changelog_generator.

But a lot of extensions do not have that metadata set. We can be more kind by not making specs ran with rake (which are unrelated) fail when that information is missing.

This is the output when that metadata is missing after this PR will be merged:

$ bundle exec rake
It won't be possible to automatically generate the CHANGELOG for this extension because the
gemspec is missing the `source_code_uri` metadata. Please add this line to the gemspec to
enable automatic CHANGELOG generation:

    s.metadata["source_code_uri"] = 'https://github.com/org/repo'

Refs:

• https://github.com/solidusio-contrib/solidus_globalize/issues/121#issuecomment-760102482
• https://github.com/solidusio-contrib/solidus_redirector/pull/7

Checklist

• [x] I have structured the commits for clarity and conciseness.
• [ ] ~I have added relevant automated tests for this change.~

[mergify[bot]]

It looks like this PR is missing a label to determine the type of change it introduces. The maintainer should add one of the following labels:

• bug for bugfixes.
• enhancement for new features and improvements.
• documentation for documentation changes.
• security for security patches.
• removed for feature removals.
• infrastructure for internal changes that should not go in the changelog.

Additionally, the maintainer may also want to add one of the following:

• breaking for breaking changes.
• deprecated for feature deprecations.

Once the correct labels have been set, simply remove the needs changelog label label from this PR so I can merge it.