fix(fcm): Fix error throw when data property exists but is undefined

[italoiz]

When use messaging().sendToDevice(tokens, payload) where payload contains the data property, but it's contains undefined value, I given throws the follow error:

TypeError: Cannot use 'in' operator to search for 'from' in undefined

The following Pull Request contains the solution of the issue with the covered unit tests.

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[italoiz]

@googlebot I signed it!

[aprilmintacpineda]

This has recently happened to me.

[italoiz]

Hey @hiranya911, Is this PR still need a fix?