Fix NaN replace with `pandas==1.3.0`

[mmaelicke]

Initializing new databses does not work with pandas>=1.3.0 anymore. This line is causing the trouble:

https://github.com/VForWaTer/metacatalog/blob/e9634a950c6e760ff0803ff1ec65aa77df69eebd/metacatalog/api/db.py#L132

Stackoverflow issue:

• https://stackoverflow.com/questions/14162723/replacing-pandas-or-numpy-nan-with-a-none-to-use-with-mysqldb

This might solve the problem: https://stackoverflow.com/questions/14162723/replacing-pandas-or-numpy-nan-with-a-none-to-use-with-mysqldb/54403705#54403705

Todos:

• [x] check the solution works with pandas >= 1.3.0
• [x] change requirements.txt to allow pandas 1.3.0 again