Small fix to MaybeGetMinimumShape() in op_level_cost_estimator.

(1) previously, it set unknown shape flag for scalar input, but now it
returns TensorShapeProto with rank equal to the expected and all dims set to 1,
and unknown shape flag is not set.

(2) Also, fixed a bug; when a rank is known, but dim_size() < rank (note that
dim_size() may be non-zero), we previously called add_dim() with dim 1 rank
times, which then makes dim_size() is incremented by rank, but we expect
dim_size() equal to rank.

(3) Added test for MaybeGetMinimumShape().

PiperOrigin-RevId: 208845501