Handle null value in typechecker

[jdidion]

See discussion w/ Patrick here: openwdl/wdl#361

It is part of the 1.0 spec that null is a valid value in meta sections, but Patrick argues that this should be handled as a special case by tools. So the parser recognizes null as an identifier, but the typechecker should treat null as a valid identifier within the meta sections even if there is no declaration for an input/variable named 'null'.

[orodeh]

Implemented and on master.