Fix build of affine load/store with empty map

tensorflow/mlir#58 fixed and exercised
verification of load/store ops using empty affine maps. Unfortunately,
it didn't exercise the creation of them. This PR addresses that aspect.
It removes the assumption of AffineMap having at least one result and
stores a pointer to MLIRContext as member of AffineMap.

* Add empty map support to affine.store + test
* Move MLIRContext to AffineMapStorage

Closes tensorflow/mlir#74

PiperOrigin-RevId: 264416260