Add support for incomplete shapes

For example, `shape=(None, 3)` would mean any 2-d array where the second dimension is 3

The issue gives the intended example, `shape=(None, 3)`, meaning any two-dimensional array whose second dimension is 3. Start by locating the shape-validation entry point in the repository and check its existing shape tests. Done means incomplete dimensions are accepted with the stated constraint while existing shape validation remains correct.

Written by the indexing model from the issue text.