Don't assume manifest will be set in the nextflow config.

[ewels]

Fixes #1418

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[sateeshperi]

lgtm

[codecov]

Codecov Report

Merging #1449 (e315342) into dev (eb35d3a) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev    #1449   +/-   ##
=======================================
  Coverage   64.81%   64.81%           
=======================================
  Files          51       51           
  Lines        5977     5977           
=======================================
  Hits         3874     3874           
  Misses       2103     2103           

Impacted Files	Coverage Δ
nf_core/modules/module_utils.py	38.23% <0.00%> (ø)

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