This fixes firebase/firebase#19004

[SPodjasek]

There was some bug introduced possibly in firebase messaging package version 17.1.0 which was described in:

• FirebaseMessaging: Error while parsing timestamp in GCM event flutter#19004
• NullPointer Exception in Parsing Error while parsing timestamp in GCM event : FireBaseMessaging firebase/quickstart-android#588

upgrading to com.google.firebase:firebase-messaging:17.3.0 fixed it.

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[SPodjasek]

I've signed it!

[googlebot]

CLAs look good, thanks!

[bkonyi]

Ah shoot, didn't see this before merging in #737 which is the same fix. Thanks for the PR either way. We appreciate it!