lookups doesn't accept null

[devkral]

when defining a query filter like:

query {
  users(filter: {username: {contains: null}}){
  }
}

the query fails with with: "Cannot use None as a query value" except for some filters like exact. And for exact this is a spec violation:

filters shall accept Unset or None to not filter

This bug causes errors when testing with schemathesis. PR follows

System Information

• Strawberry version (if applicable): 0.29

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