Switch equal to a safer equal or less check

This is to cover cases with potential signed integers in the future.

contracts/solidity/contracts/StakeDelegatable.sol

@@ -24,7 +24,7 @@ contract StakeDelegatable {
      * @dev Only non staker addresses can be passed to the functions with this modifier.
      */
     modifier notStaker(address _address) {
-        require(stakeBalances[_address] == 0, "Provided address is not a staker.");
+        require(stakeBalances[_address] <= 0, "Provided address is not a staker.");
         _;
     }