Cast result of getContributionBalance to float to match comment block.

Without this it is possible for it to return 'null' which is not useful / in keeping. I did a bit of an audit &
did not find places where this would cause an === comparison to fail

CRM/Contribute/BAO/Contribution.php

@@ -4166,7 +4166,7 @@ public static function getContributionBalance($contributionId, $contributionTota
       $contributionTotal = CRM_Price_BAO_LineItem::getLineTotal($contributionId);
     }
 
-    return CRM_Utils_Money::subtractCurrencies(
+    return (float) CRM_Utils_Money::subtractCurrencies(
       $contributionTotal,
       CRM_Core_BAO_FinancialTrxn::getTotalPayments($contributionId, TRUE) ?: 0,
       CRM_Core_DAO::getFieldValue('CRM_Contribute_DAO_Contribution', $contributionId, 'currency')