WIP Fix Exception when trying to analyze custom attribute that is being defined in different file

[bergmeister]

PR Summary

Fixes #966
This is a quick fix. I would like to understand more details of it. If the definition is not part of a file being analysed by PSSA or know to PowerShell then we cannot do anything. But maybe I can improve the overall pre-analysis or something else.
A test is to be written.

PR Checklist

Note: Tick the boxes below that apply to this pull request by putting an x between the square brackets. Please mark anything not applicable to this PR NA.

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• User facing documentation needed -> Maybe tell the user to load as many definitions as possible into the PowerShell sessions first
• Change is not breaking
• To be done. Make sure you've added a new test if existing tests do not effectively test the code changed
• This PR is ready to merge and is not work in progress
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready

[hubuk]

May be closed as handled in #1659