Throw when providing null or empty array to -ForEach

[fflaten]

PR Summary

Throw during Discovery when Describe/Context/It is provided $null or @() to -ForEach/TestCases to avoid silent failure.

The previous behavior, ignoring the test/block, can be achieved by setting the configuration option Run.FailOnNullOrEmptyForEach to $false.

When enabled, individual blocks or tests can be overriden to allow empty array or null by adding a new -AllowNullOrEmptyForEach parameter to Describe/Context/It, e.g. if a test is generated using external data which might be empty.

Fix #2151

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[fflaten]

@nohwnd It already paid off 😄 a365340

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[nohwnd]

Amazing work!