Fix minimal modules bug

The minimal modules bug on PrgEnv-cray was due to an empty array
initializer list with an inferred size, which is apparently illegal.
The fix was as simple as printing out a single null pointer in the
list in this case (something that we already did for a previous
list -- I simply mimicked the logic).
Merge branch 'master' of https://github.com/chapel-lang/chapel into fix-minmodules-empty-vmt
Merge branch 'master' of https://github.com/chapel-lang/chapel into fix-minmodules-empty-vmt
Merge pull request #841 from bradcray/fix-minmodules-empty-vmt

Fix empty virtual method tables in generated code in --minimal-modules mode

[reviewed by noakes]

It seems that certain compilers who shall remain nameless feel that throwing an error for an assumed-size C array with an empty initializer is justified (e.g.,

int x[] = {};

but that unsupported inlined assembly sequences are only worthy of a warning. Whatevs. It turns out that the --minimal-modules flag runs afoul of this by creating an empty virtual method table. The fix is simply to drop a single NULL pointer into the initializer list in this case (we had a similar fix somewhat higher in this file for another array that is much more likely to be empty in traditional compilations, so I essentially took that same approach here, simply with a different trigger condition).