updating column units from "NaN" to "" errors out

[yang-ruoxi]

MPContribs/mpcontribs-client/mpcontribs/client/__init__.py

Line 1526 in 13c97a5

conv_args.append(ureg.Unit(u))

When updating the column units from "NaN" to "" (dimensionless), one will get the ValueError: "Can't convert NaN to for {path}".
The reason is that ureg.Unit(u) can't take "NaN" as value.
Add a u.lower() != "nan" would address it.