WARN: Only warn about inconsistent parsing if there are multiple non-null elements
- [ ] closes #52167 (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Not sure this needs a whatsnew note as it could just be considered part of the PDEP4 changes, which haven't yet reached user-facing status

The idea here is to only warn if there are at least 2 non-null elements - otherwise, a single element can't be inconsistently-parsed :) 
Thanks @MarcoGorelli 