fix(empty transient data): Empty first stress period block (#1091)

[spaulins-usgs]

Empty stress period 1 transient data is now retained and used to create empty stress period block (#1091)

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 71.3%. Comparing base (4c44cb0) to head (53147d0).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2139     +/-   ##
=========================================
+ Coverage     71.1%   71.3%   +0.1%     
=========================================
  Files          260     260             
  Lines        58076   59491   +1415     
=========================================
+ Hits         41341   42443   +1102     
- Misses       16735   17048    +313     

Files	Coverage Δ
flopy/mf6/mfpackage.py	64.8% <100.0%> (+0.1%)	⬆️
flopy/mf6/data/mfdatalist.py	62.4% <33.3%> (+<0.1%)	⬆️

... and 85 files with indirect coverage changes