prevent metpy issue with all nans

[codecov[bot]]

Codecov Report

Base: 80.03% // Head: 79.96% // Decreases project coverage by -0.07% :warning:

Coverage data is based on head (9df3301) compared to base (4a23f87). Patch coverage: 66.66% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #128 +/- ## ========================================== - Coverage 80.03% 79.96% -0.07% ========================================== Files 61 61 Lines 1963 1967 +4 ========================================== + Hits 1571 1573 +2 - Misses 392 394 +2 ``` | [Impacted Files](https://codecov.io/gh/akrherz/iem-web-services/pull/128?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daryl+herzmann) | Coverage Δ | | |---|---|---| | [src/iemws/services/obhistory.py](https://codecov.io/gh/akrherz/iem-web-services/pull/128?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daryl+herzmann#diff-c3JjL2llbXdzL3NlcnZpY2VzL29iaGlzdG9yeS5weQ==) | `65.95% <66.66%> (-0.71%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daryl+herzmann). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daryl+herzmann)

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