Delay allocating variationMaps until populating them

Modules are created with a nil variantionMap, which avoids allocating
an empty map if the module will never be split.
Context.addVariationDependencies constructs a variationMap to
compare against the variationMap of each variant, but constructed
an empty variationMap if no variations were specified.  A nil
map is not the same as an empty map, so consistently use a nil
map and create it when populating the first entry.

Change-Id: I48b604659f9cdb23326b504a093cdfe5a3eb4f68