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[bot]]

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

```diff @@ 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](https://app.codecov.io/gh/modflowpy/flopy/pull/2139?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy) | Coverage Δ | | |---|---|---| | [flopy/mf6/mfpackage.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2139?src=pr&el=tree&filepath=flopy%2Fmf6%2Fmfpackage.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L21mcGFja2FnZS5weQ==) | `64.8% <100.0%> (+0.1%)` | :arrow_up: | | [flopy/mf6/data/mfdatalist.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2139?src=pr&el=tree&filepath=flopy%2Fmf6%2Fdata%2Fmfdatalist.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L2RhdGEvbWZkYXRhbGlzdC5weQ==) | `62.4% <33.3%> (+<0.1%)` | :arrow_up: | ... and [85 files with indirect coverage changes](https://app.codecov.io/gh/modflowpy/flopy/pull/2139/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy)