[MEX-514] fix compute user APR if no total staked amount

[claudiulataretu]

Reasoning

• if user doesn't have any total staked amount registered, the query would return an error for non-nullable field
• if user doesn't have any total farm amount registered, the query would return an error for non-nullable field

Proposed Changes

• check for user total staked amount when computing boosted APR for staking
• check for user total farm amount when computing boosted APR for farms

How to test

query StakingBoostedRewards {
    getStakingBoostedRewardsBatch(
        stakingAddresses: [
            <staking_address>
        ]
    ) {
        curentBoostedAPR
        maximumBoostedAPR
    }
}

• query should return 0 for both fields if no total staked amount for user