[FIX] handle null values in metadata

[jdkent]

references neurostuff/neurostore#587 .

Changes proposed in this pull request:

• allows the manipulation of sample_size meta-data when it contains null values

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (0864fd2) 88.67% compared to head (cfda23d) 88.67%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #831   +/-   ##
=======================================
  Coverage   88.67%   88.67%           
=======================================
  Files          47       47           
  Lines        6020     6020           
=======================================
  Hits         5338     5338           
  Misses        682      682           

Files Changed	Coverage Δ
nimare/utils.py	94.19% <100.00%> (ø)

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