Fix missing return in _retryer that propagated None tiles

[madhavcodez]

Root cause

_retryer retries on non-404 failures, but the retry branch never returned the recursive call's result:

request = _retryer(tile_url, wait, max_retries, headers, timeout=timeout)

After kicking off the retry, the function fell through and implicitly returned None. That None was collected as a tile "array" and later reached _merge_tiles, where h, w, d = arrays[0].shape raised AttributeError: 'NoneType' object has no attribute 'shape' (or a TypeError on the array assignment when the None was not in first position). This is why #252 was intermittent: it only surfaced when a tile hit a non-404 error and then succeeded on retry. The earlier np.asarray(None) theory in the thread was not the cause.

Fix

• Return the recursive call in _retryer (request = _retryer(...) becomes return _retryer(...)). This is the actual fix.
• Defense in depth: _merge_tiles now raises a clear ValueError if any tile array is None, instead of failing with an obscure AttributeError/TypeError further down.

Tests

Three network-free tests added: _merge_tiles raises the clear ValueError for a missing tile in both first and later positions; _retryer returns the array (not None) after a non-404 failure followed by a successful retry; and _retryer raises (rather than returning None) when retries are exhausted. They pass with the fix and fail without it. pytest -m "not network" reports 14 passed.

The second change (the _merge_tiles guard) is defense in depth and not required to fix #252. Happy to drop it if you would prefer the root-cause change only.

Closes #252