Fix CodeBase for existing inproc collector settings

[MarcoRossignoli]

In-proc coverlet collector is injected by collect arg processor.
If no coverlet in-proc setting is found full xml element properties setting is injected.
If coverlet in-proc setting is found we ensure to setup mandatory field AssemblyQualifiedName CodeBase IsEnabled
Past fix to avoid custom code to pass adapters path to host fixed "default" and most common case where no in-proc element is present #2288

This PR fix the case where in-proc element is present and codebase is null.
Check for codebase if null search using adapter path if null setup default coverlet codebase CoverletConstants.CoverletDataCollectorCodebase

BTW after #2288 in-proc collector won't be injected because coverlet nuget adapter path won't be passed to testhost.

In conclusion...to work coverlet needs adapter path loaded in TestAdaptersPaths.

cc: @nohwnd @jakubch1 @vagisha-nidhi

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