Fix bug constructing RDom of undefined ReductionDomain
Fix bug constructing RDom of undefined ReductionDomain

It now creates an undefined RDom instead of crashing


Former-commit-id: 536fad31efa5c8007c776903f33e7715571b007f
Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide