[MLIR] Fix TestAffineDataCopy for test cases with no load ops

Add missing check in -test-affine-data-copy without which a test case
that has no affine.loads at all would crash this test pass. Fix two
clang-tidy warnings in the file while at this. (Not adding a test case
given the triviality.)

Differential Revision: https://reviews.llvm.org/D101719