chore: addressing a few PR comments (#446)

Author: ymc9

packages/orm/src/client/contract.ts

@@ -61,6 +61,9 @@ export enum TransactionIsolationLevel {
  * ZenStack client interface.
  */
 export type ClientContract<Schema extends SchemaDef, Options extends ClientOptions<Schema> = ClientOptions<Schema>> = {
+    /**
+     * The schema definition.
+     */
     readonly $schema: Schema;
 
     /**

packages/orm/src/utils/schema-utils.ts

@@ -81,6 +81,7 @@ export class MatchingExpressionVisitor extends ExpressionVisitor {
     }
 
     find(expr: Expression) {
+        this.found = false;
         this.visit(expr);
         return this.found;
     }

packages/plugins/policy/src/expression-evaluator.ts

@@ -106,7 +106,7 @@ export class ExpressionEvaluator {
         invariant(op === '?' || op === '!' || op === '^', 'expected "?" or "!" or "^" operator');
 
         const left = this.evaluate(expr.left, context);
-        if (!left) {
+        if (left === null || left === undefined) {
             return null;
         }