[Fix] Error running rank-0 tensor conformance test

This seg fault error can be reproduced by creating a custom SUT
testing with scalars (rank-0 tensor) e.g. np.half(1), or np.array(1).
Iterating over the shape, which is empty for a scalar, is invalid.

Change-Id: I6b0677baf8ff40cb13c146c91775bc28fd22cc9b
Signed-off-by: TatWai Chong <tatwai.chong@arm.com>