Allow NullExp as struct hidden member in struct literals

Since CTFE will create null hidden pointers.
Fix bug 10217 ICE(root.c) with nested struct.init.tupleof in CTFE

CTFE version of bug 9315, almost exactly the same fix.
If a nested struct has no initialized hidden pointer, set it to null
to match the runtime behaviour.
Merge pull request #2107 from donc/ctfe_10217_ice_nestedstruct

10217: ICE(root.c) with nested struct.init.tupleof in CTFE