V2ServerAPICalls: Add `searchTerm=''` param for JFrog Artifactory calls

[sean-r-williams]

PR Summary

This PR adds an empty searchTerm request parameter to Artifactory calls where searchTerm is not otherwise provided.

Resolves #1531.  

PR Context

Based on investigation in #1485 and #1531, on-premises Artifactory installations do not enumerate the contents of a feed when a v2 Search() request does not include a searchTerm parameter.

PowerShellGet previously included searchTerm='' on find-all requests - this PR restores that behavior in this specific scenario.

Depending on the PowerShell team's preferences, it may be worthwhile to include this for any provider instead of just Artifactory (i.e. remove the ternary conditional on _isJFrogRepo).

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