Fix no-unnecessary-qualifier handling of implicit arguments

[GoodForOneFare]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Symbol#getDeclarations returns null when it's describing an arguments reference. This change short-circuits symbolIsNamespaceInScope if declarations are null.

Is there anything you'd like reviewers to focus on?

Nope.

CHANGELOG.md entry:

[bugfix] no-unnecessary-qualifier: no longer breaks when walking a function that references arguments

[adidahiya]

thanks @GoodForOneFare!