recGblInitConstantLink broken in 3.16

[Michael Davidsaver <mdavidsaver@bnl.gov>]

I broke it (to avoid the suspense).  Only 3.16 branch is effected.

It seems that my recent link field parsing changes have broken
initialization of VAL in output records (really anything which call
recGblInitConstantLink()).

I missed a subtle distinction between constantStr==NULL and
constantStr=="" during initialization.  recGblInitConstantLink() treats
NULL as a no-op, and "" as equivalent to "0".

This comes about if a link field definition provides an initial() value.
 (see dbAllocRecord()).

This breaks non-default values in .db files and autosave pass0.

Test case and fix are forthcoming.