Allow empty string for Should-BeString -Expected

[nohwnd]

Fix #2857

Problem

Should-BeString rejects an empty string for -Expected:

"" | Should-BeString ""
# Cannot bind argument to parameter 'Expected' because it is an empty string.

This forces users porting v5 -ForEach tests to branch between Should-BeString and Should-BeEmptyString depending on whether the expected value is empty.

Root cause

The rejection is not a deliberate assertion rule — there's no validation in the function body checking for empty. It's an implicit side effect of declaring the parameter as [Parameter(Mandatory)][String]$Expected: a mandatory, typed [String] parameter automatically refuses "" during parameter binding.

Everything downstream already handles empty strings:

• Test-StringEqual compares empty strings correctly.
• Get-StringDifferenceMessage already declares [AllowEmptyString()] on $Expected/$Actual.
• The sibling Should-NotBeString doesn't mark Expected as Mandatory, so it already accepts empty strings — Should-BeString was the inconsistent one.

Change

• Add [AllowEmptyString()] to $Expected (keeping Mandatory, so the argument is still required — it may just be empty).
• Add a comment-based-help example.
• Add tests covering: both empty (named, positional, pipeline) and the failure/diff message when expected is "" but actual is non-empty.

All 42 tests in Should-BeString.Tests.ps1 pass.