docs: warn for no mass

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1302d99) 77.07% compared to head (294e7c5) 77.08%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #125 +/- ## ======================================= Coverage 77.07% 77.08% ======================================= Files 113 113 Lines 16502 16503 +1 Branches 3103 3103 ======================================= + Hits 12719 12721 +2 Misses 3074 3074 + Partials 709 708 -1 ``` | [Flag](https://app.codecov.io/gh/jiangyi15/tf-pwa/pull/125/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jiang+Yi+%28%E8%92%8B%E8%89%BA%29) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/jiangyi15/tf-pwa/pull/125/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jiang+Yi+%28%E8%92%8B%E8%89%BA%29) | `77.08% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jiang+Yi+%28%E8%92%8B%E8%89%BA%29#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/jiangyi15/tf-pwa/pull/125?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jiang+Yi+%28%E8%92%8B%E8%89%BA%29) | Coverage Δ | | |---|---|---| | [tf\_pwa/amp/core.py](https://app.codecov.io/gh/jiangyi15/tf-pwa/pull/125?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jiang+Yi+%28%E8%92%8B%E8%89%BA%29#diff-dGZfcHdhL2FtcC9jb3JlLnB5) | `76.88% <100.00%> (+0.01%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/jiangyi15/tf-pwa/pull/125/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jiang+Yi+%28%E8%92%8B%E8%89%BA%29)

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