Fix Clear-PnPDefaultColumnValues not working with taxonomy fields

[jackpoz]

Clear-PnPDefaultColumnValues currently throws a NullDereferenceException when invoked on taxonomy fields.
This PR fixes the exception caused by GetDefaultColumnValueFromField() returning null when an empty array is passed as argument (meaning "no default taxonomy values"), returning instead a DefaultColumnTermValue object with no terms in it, and changes SetDefaultColumnValuesImplementation to clear any default column value if there are none left to be kept.

In addition to this I added a null check in ListExtensions.ClearDefaultColumnValues() as it was triggering a null exception when the folder "Forms" was not present (I didn't investigate further if this happens only in special cases, like modern lists just provisioned).

Fixes pnp/powershell#1110 , https://github.com/MicrosoftDocs/OfficeDocs-SharePoint-PowerShell/issues/130 , pnp/PnP-PowerShell#1978

Tests done:

1. Add a default taxonomy value, then remove it and expect no default values left
   

2. Add a default taxonomy value and a default string value, then remove the taxonomy and expect the string value to still be there. Remove the string value too and expect no default values left
   

3. Apply a PnP Template with some lists and different column types