RTDB exists() should return true for falsy values (e.g. 0, false, "") (#1410)

Fix for #1407

Author: inlined

Diff for: spec/v1/providers/database.spec.ts

@@ -580,6 +580,15 @@ describe("DataSnapshot", () => {
       populate({ a: [{}] });
       expect(subject.child("a").exists()).to.be.false;
     });
+
+    it("should be true for a falsy value (other than null)", () => {
+      populate({ num: 0, bool: false, n: null });
+      expect(subject.exists()).to.be.true;
+      expect(subject.child("num").exists()).to.be.true;
+      expect(subject.child("bool").exists()).to.be.true;
+      expect(subject.child("n").exists()).to.be.false;
+      expect(subject.child("missing").exists()).to.be.false;
+    });
   });
 
   describe("#forEach(action: (a: DataSnapshot) => boolean): boolean", () => {

Diff for: src/common/providers/database.ts

@@ -159,7 +159,7 @@ export class DataSnapshot implements database.DataSnapshot {
    */
   exists(): boolean {
     const val = this.val();
-    if (!val || val === null) {
+    if (typeof val === "undefined" || val === null) {
       return false;
     }
     if (typeof val === "object" && Object.keys(val).length === 0) {