Fix VFA validation for NumPy flip angles

[ParikshitGupta2605]

Summary

This PR fixes the validation of flip_angles in compute_t1_vfa() when provided as a NumPy array.

Previously, the code used:

if not params.flip_angles:

which raises a ValueError for NumPy arrays because their truth value is ambiguous.

Changes

• Replace the boolean truth-value check with explicit validation for None and empty arrays.
• Convert flip_angles using np.asarray() before validation.
• Add a regression test to ensure NumPy arrays are accepted.

Fixes #159