Fix create_partitions sending empty assignments array instead of null (#3146)

[ebrard]

Fixes #3146.

Problem

create_partitions({topic: N}) with a plain int total count — the form the docstring recommends — fails against a normal broker with InvalidReplicationAssignmentError (error 39):

kafka.errors.InvalidReplicationAssignmentError: [Error 39]
  CreatePartitionsTopic(name='demo-topic', count=6, assignments=[])
  error_message='Attempted to add 3 additional partition(s), but only 0 assignment(s) were specified.'

create_partitions({topic: NewPartitions(N)}) (auto-assign, no manual assignments) is also broken — it raises TypeError: 'NoneType' object is not iterable, because the branch iterates count.new_assignments unconditionally even though it defaults to None.

Root cause

_process_create_partitions_input always built the per-topic assignments field as a list. For the int form (and a bare NewPartitions) that yields an empty list [], which serialises to a present-but-empty array rather than null. The broker reads a present array as "manual replica assignment" and rejects it because the assignment count doesn't match the number of new partitions.

Verified at the byte level (assignments field only, after count=00000006):

# v3 (compact array length):
[]   -> ...00000006 01 ...   # length 1 => 0 elements, present array
None -> ...00000006 00 ...   # 0 => null

# v0 (classic 4-byte array length):
[]   -> ...00000006 00000000 ...   # length 0, present
None -> ...00000006 ffffffff ...   # -1 => null

null means "auto-assign replicas"; a present array means "manual assignment". The empty list is what triggers error 39.

Fix

Pass assignments=None when the caller supplies no manual assignments, so it serialises to null and the broker auto-assigns. Manual assignments (dict form and the deprecated NewPartitions form) are preserved unchanged.

Minor related change: the dict branch now uses count.get('assignments') instead of count['assignments'], so {'count': N} with no assignments means auto-assign rather than raising KeyError. This is backward compatible (any dict that previously worked still works).

Tests

Added unit coverage in test/admin/test_admin_topics.py asserting _process_create_partitions_input produces assignments is None for the int and bare-NewPartitions inputs, and preserves explicit manual assignments. The auto-assign test fails on the current code and passes with this change.

Verified:

• test/admin/test_admin_topics.py — 20 passed (incl. the 2 new tests)
• test/protocol/admin/test_protocol_admin.py — 121 passed
• End-to-end against a live confluentinc/cp-kafka:latest broker: create_partitions({topic: N}) (int) now increases the partition count instead of raising error 39, confirmed with rpk topic describe.

[dpkp]

Thanks; I think the fix is much simpler than this -- we can just update the default to 'null'. However, the tests are much appreciated!

[dpkp]

Updated in #3148