not rejecting coercion of some optional types to (non-optional) String

[mlin]

SEE: https://github.com/broadinstitute/cromwell/issues/6840

We are allowing coercion of some optional primitive types (File? Directory? Boolean? etc.) to non-optional String, but this seems indefensible when we do not allow String? to String coercion.

Investigate if there's some history here, e.g. do a bunch of tests break if we tighten this? Even if not, we should take our time with a potentially breaking change -- start with a linter warning for some months.