Add missing return to stop plugin in case of empty input

[HenningSE]

What does the code in this PR do / what does it improve?

This PR fixes a problem with empty input chunks to the PMTResponseAndDAQ plugin. Before this fix, the iteration for empty chunks was not properly stoped leading to data not continuous problems.

Can you briefly describe how it works?

I added the missing return to the empty input case of the PMTResponseAndDAQ plugin.

Please include the following if applicable:

• Bump plugin version(s)
• Tests to check the (new) code is working as desired.

[coveralls]

Pull Request Test Coverage Report for Build 9107702936

Details

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 71.482%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
fuse/plugins/pmt_and_daq/pmt_response_and_daq.py	1	2	50.0%

Totals
Change from base Build 9107691937:	-0.02%
Covered Lines:	2108
Relevant Lines:	2949

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💛 - Coveralls