fix: Exclude votes without proposals

[ChaituVR]

Temporary fix for #760

How to test:

• Test with:

query Votes {
    votes(first: 1000, where: { voter: "0x24F15402C6Bb870554489b2fd2049A85d75B982f" }) {
     id  
    proposal {
        id
        space {
          id
        }
        ipfs
      }
    }
  }
  

Should return values without null in proposal

query { 
  vote(id: "0xd954ffa9c8766c7f1195f867522ac759d8f752781d811941c91e289032e7df90") {
    
    id
    proposal {
      id
    }
  }
}

Should return null

[wa0x6e]

utAck