Remove code that aborts the result builder transform when we encounter
a case that has no statements in it. This can occur when the only
statements were behind a `#if` that evaluataed empty, so it should not
cause an abort.
Previously, the presence of an IfConfigDecl within the case statement
would have prevented us from aborting the traversal here. However, the
removal of IfConfigDecl from the AST turned this previously-accepted
code into a compiler crash.
Fixes rdar://139312426.
