[FEATURE] Support invalid / empty integer values

[kreisligaspieler]

User Story

1. As a user
2. I want to import rows with empty cells into a database
3. So that i can have empty values not only in text columns, but also in integer columns

User Acceptance Criteria

• By default rows whit empty values in integer columns are dropped. It would be better if the user could choose between dropping columns or assigning a "Null"-Value.

Examples

https://download-data.deutschebahn.com/static/datasets/haltestellen/D_Bahnhof_2020_alle.CSV
-> "Betreiber_Nr" is sometimes empty, so at them moment the corresponding rows are skipped during the import. A solution would be to change the definition of integer or to allow something like that: "Betreiber_Nr" oftypeype integer | undefined

[georg-schwarz]

Will be incorporated in #291
I will close the issue to avoid duplications.

Thanks for the contribution!