Fixed issue calling DataSnapshot methods with null data

[rhodgkins]

Description

Fixes firebase/firebase-functions-test#254

Fix is similar to the check in #exists():

firebase-functions/src/common/providers/database.ts

Line 173 in 5f7b331

if (typeof val === "undefined" || val === null) {

[inlined]

@taeold IIUC this is a straight bugfix, but I wanted to double check that you don't consider this a breaking change.

[taeold]

Feels like this is a good change and not breaking.

IIUC the change would only break if the traversing the full path instead of short-circuiting somehow had a side effect that was worthy.

[rhodgkins]

@taeold any ideas on the timeline when this might be merged / released?

[rhodgkins]

Sorry I missed that there were tests in the project for this! I've added some tests for the issue to make it clearer.
I've also update the fix as adding tests exposed another issue when this._data is directly null (and not as a result of a child being missing or null).

[rhodgkins]

As well as moving this "up" to the parent object I've also changed the check from === undefined to a typeof so it matches the other checks elsewhere:

firebase-functions/src/common/providers/database.ts

Line 173 in 5f7b331

if (typeof val === "undefined" || val === null) {

firebase-functions/src/common/providers/database.ts

Line 278 in 5f7b331

if (node === null || typeof node === "undefined") {