Do not throw on PSGallery

[kingsleyck]

Pull Request (PR) description

Do not throw when PSGallery does not exist and SourceLocation is unspecified.

This Pull Request (PR) fixes the following issues

None

Task list

• [ ] Added an entry under the Unreleased section of the change log in the CHANGELOG.md. Entry should say what was changed, and how that affects users (if applicable).
• [ ] Resource documentation added/updated in README.md.
• [ ] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [ ] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [x ] Unit tests added/updated. See DSC Resource Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• [x ] New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

[codecov-io]

Codecov Report

Merging #2 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master       #2   +/-   ##
=======================================
  Coverage   94.49%   94.49%           
=======================================
  Files           2        2           
  Lines         109      109           
=======================================
  Hits          103      103           
  Misses          6        6

Impacted Files	Coverage Δ
...ces/PowershellRepository/PowershellRepository.psm1	90.32% <100%> (ø)	:arrow_up:

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