fuse.py: Prevent division by zero error.

If no read was segmented we may encounter an error in that case
7 jobs for feature-tc/3943-implement-null-for-normalized_reads in 7 minutes and 33 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test Tools
passed #367044
test_tools

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  Test Quality
passed #367045
web
code_quality

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  Test Unit
passed #367046
web
test_browser_unit

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  Deploy Review
passed #367047
web
deploy_review

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passed #367048
web allowed to fail manual
stop_deploy_review

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  Test Functional
passed #367049
web
test_browser-functional

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  Test Functional External
passed #367050
web
test_browser-functional-external

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