`Should-BeString` should allow empty strings for the `-Expected` parameter

[CedX]

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I've this kind of v5 test:

It "should do something" -ForEach @(
	@{ Value = ""; Length = 0; Expected = "" }
	@{ Value = "foo bar"; Length = 3; Expected = "foo" }
) {
	Format-String $value -Truncate -Length $length | Should -BeExactly $expected
}

When porting it to v6, I now must use:

It "should do something" -ForEach @(
	@{ Value = ""; Length = 0; Expected = "" }
	@{ Value = "foo bar"; Length = 3; Expected = "foo" }
) {
	$actual = Format-String $value -Truncate -Length $length
	if ($expected) { Should-BeString $expected $actual -CaseSensitive }
	else { Should-BeEmptyString $actual }
}

I would have preferred not to have to use two different assertions depending on whether the expected value is an empty string or not.