Fix cache of Rechunker, zero chunk is not None

[dachengx]

What is the problem / what does the code in this PR do

This PR solves the error we see in straxen testing GitHub action: https://github.com/XENONnT/straxen/actions/runs/9516464741/job/26232816323

When the length of a chunk is zero, this line returns False:

strax/strax/chunk.py

Line 448 in d92e64b

if self.cache:

Which is not intended. The intended behavior is to concatenate the cache and a new chunk if the cache is not None. This PR fix it.

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[github-actions]

coverage: 90.458% (+0.2%) from 90.238%
when pulling 2f1c6b8 on debug_chunk
into d92e64b on master.

[coveralls]

coverage: 90.458% (+0.2%) from 90.238%
when pulling 2f1c6b8 on debug_chunk
into d92e64b on master.

[coveralls]

coverage: 90.519% (-0.07%) from 90.587%
when pulling 2f1c6b8 on debug_chunk
into d92e64b on master.

[coveralls]

Changes unknown
when pulling 2f1c6b8 on debug_chunk
into ** on master**.