Fix parser exception in using statements with empty aliases

[MartinGC94]

PR Summary

Fixes #16676

The parser threw an exception when entering a new line after the equals token, and when you entered a comma after the equals sign like in these 2 examples:

using namespace whatever =

using namespace whatever = ,

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[iSazonov]

@andschwa Have you any concerns about PSES?

[SeeminglyScience]

LGTM! No PSES concerns I can think of.

[andyleejordan]

I agree with @SeeminglyScience, I don't see an impact on PSES.

[MartinGC94]

I also discovered that commas have the same problem: using command X = , causes an error.

[iSazonov]

@MartinGC94 Please look test error.

[iSazonov]

@MartinGC94 I reviewed all code paths with GetCommandArgument() and found the same issue if we type configuration ,.
It is in private ExpressionAst GetWordOrExpression(Token keywordToken). There is GetCommandArgument() call with only TokenKind.EndOfInput check.

It would be great if you fixed this too.

[MartinGC94]

I will take a look later, but typing configuration , into the console doesn't cause an exception on my system so that scenario may already be handled elsewhere.

[iSazonov]

Assert throw in debug build

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This PR has 31 quantified lines of changes. In general, a change size of upto 200 lines is ideal for the best PR experience!

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Label      : Extra Small
Size       : +28 -3
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Total files changed: 2

Change summary by file extension:
.cs : +13 -3
.ps1 : +15 -0

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[adityapatwardhan]

@MartinGC94 Thank you for your contribution!

[ghost]

🎉v7.3.0-preview.7 has been released which incorporates this pull request.:tada:

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