Added postgres test for union with two nulls

one column is null on both sides of the union
Removed exception swallowing from ConstantFolder
Handle select null union null correctly

there shouldn't be a cast to a null type
Fixed evaluateConstant for routines

Setting the constant result should happen outside the loop.
The specific problem was that my func too 0 parameters so it never
ran that function at all
Added union-null to readme
merge ScottDugas: Stop Constant folder exception swallowing

Merge pull request #1522 from ScottDugas/sql-layer/constant-folder-exception-swallowing
https://github.com/FoundationDB/sql-layer/pull/1522