Fix rewards snapshot not being called when POI presented is 0x0 (#459)

When an indexer calls closeAllocation() with a POI=0x0 the function distributeRewards() is not called, as a consequence, takeRewards() is not called and in turn the snapshots are not updated before unallocating tokens. This creates a situation where the rewards manager considers there are a different amount of allocated tokens and distort calculations until it is called next time.

This fix will call `_updateRewards()` when the POI presented is 0x0 before unallocating the tokens.

Author: abarmat

contracts/staking/Staking.sol

@@ -1195,6 +1195,8 @@ contract Staking is StakingV2Storage, GraphUpgradeable, IStaking {
         // Distribute rewards if proof of indexing was presented by the indexer or operator
         if (isIndexer && _poi != 0) {
             _distributeRewards(_allocationID, alloc.indexer);
+        } else {
+            _updateRewards(alloc.subgraphDeploymentID);
         }
 
         // Free allocated tokens from use