Fix Dask worker memory limit warning

[johnwlambert]

Only override memory limit if specified explicitly.

Fixes the following:

2023-08-14 22:50:57,818 - distributed.worker_memory - WARNING - Worker is at 85% memory usage. Pausing worker.  Process memory: 1.14 GiB -- Worker memory limit: 1.33 GiB
2023-08-14 22:50:57,818 - distributed.worker_memory - WARNING - Unmanaged memory use is high. This may indicate a memory leak or the memory may not be released to the OS; see https://distributed.dask.org/en/latest/worker.html#memtrim for more information. -- Unmanaged memory: 1.14 GiB -- Worker memory limit: 1.33 GiB
2023-08-14 22:50:57,820 - distributed.worker_memory - WARNING - Unmanaged memory use is high. This may indicate a memory leak or the memory may not be released to the OS; see https://distributed.dask.org/en/latest/worker.html#memtrim for more information. -- Unmanaged memory: 1.15 GiB -- Worker memory limit: 1.33 GiB
2023-08-14 22:50:57,853 - distributed.worker_memory - WARNING - Unmanaged memory use is high. This may indicate a memory leak or the memory may not be released to the OS; see https://distributed.dask.org/en/latest/worker.html#memtrim for more information. -- Unmanaged memory: 1.16 GiB -- Worker memory limit: 1.33 GiB
2023-08-14 22:50:57,952 - distributed.worker_memory - WARNING - Unmanaged memory use is high. This may indicate a memory leak or the memory may not be released to the OS; see https://distributed.dask.org/en/latest/worker.html#memtrim for more information. -- Unmanaged memory: 1.17 GiB -- Worker memory limit: 1.33 GiB
2023-08-14 22:50:58,053 - distributed.worker_memory - WARNING - Unmanaged memory use is high. This may indicate a memory leak or the memory may not be released to the OS; see https://distributed.dask.org/en/latest/worker.html#memtrim for more information. -- Unmanaged memory: 1.19 GiB -- Worker memory limit: 1.33 GiB