Add non-null constraint to community logo

[MatheusBuss]

We had a problem with https://github.com/cambiatus/frontend/pull/805, where the GraphQl schema does not accept null values for communities logos, but our SQL database does.

There are tow options:

• A migration should be created to ensure that the database also doesn't accept null values on the community logo field.

OR

• We should accept null values on the GraphQl schema

[MatheusBuss]

Based on our conversation we'll update the GraphQl schema so that the logo field is nullable. On the fronted we'll have null values replaced with a placeholder image.