Accept only bool for $exists in LiveQuery

src/LiveQuery/QueryTools.js

@@ -208,6 +208,11 @@ function matchesKeyConstraints(object, key, constraints) {
       case '$exists':
         let propertyExists = typeof object[key] !== 'undefined';
         let existenceIsRequired = constraints['$exists'];
+        if (typeof constraints['$exists'] !== 'boolean') {
+          // The SDK will never submit a non-boolean for $exists, but if someone
+          // tries to submit a non-boolean for $exits outside the SDKs, just ignore it.
+          break;
+        }
         if ((!propertyExists && existenceIsRequired) || (propertyExists && !existenceIsRequired)) {
           return false;
         }