Work around NaNs in merge_dwis

[tsalo]

Closes #942.

Changes proposed in this pull request

• Modify merge_dwis so it works if b0_mean is NaN or correction is.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 46.17%. Comparing base (6d5b1e9) to head (2c56a9c).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
qsiprep/interfaces/dwi_merge.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #968   +/-   ##
=======================================
  Coverage   46.17%   46.17%           
=======================================
  Files          65       65           
  Lines        9745     9745           
  Branches     1069     1069           
=======================================
  Hits         4500     4500           
  Misses       5022     5022           
  Partials      223      223           

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[mattcieslak]

Thank you!!