(PUP-2190) Fix issue when there are no defaults (for meta params)

When looking up a metaparameter - Notify['foo'][tag], this failed
when no tags had been set. The same problem would occur for any
parameter that the resource api reported to be a vaid parameter but
for which asking for its default did not return a Parameter.
Merge pull request #2534 from hlindberg/pup-2190_lookup-meta-parameter

(PUP-2190) Fix issue when there are no defaults (for meta params)