fix(android): Handle empty strings for allowed and disallowed VPN apps

[jamilbk]

Fixes an issue where an MDM could set these to an empty string, causing a crash because then both would be added, which isn't allowed.

https://console.firebase.google.com/u/0/project/firezone-55040/crashlytics/app/android:dev.firezone.android/issues/a79c59418b12b8de7718561aa9d23f7e?time=last-seven-days&types=crash&sessionEventKey=65E623EB03C100015D9436036031A7CF_1921042370018336672

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1 Ignored Deployment

Name	Status	Preview	Comments	Updated (UTC)
firezone	⬜️ Ignored (Inspect)			Mar 4, 2024 8:13pm

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Terraform Cloud Plan Output

Plan: 8 to add, 7 to change, 8 to destroy.

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Performance Test Results

TCP

Test Name	Received/s	Sent/s	Retransmits
direct-tcp-client2server	65.1 MiB (-2%)	65.6 MiB (-2%)	46 (+18%)
direct-tcp-server2client	55.3 MiB (+2%)	55.5 MiB (+1%)	30 (-14%)
relayed-tcp-client2server	28.9 MiB (-6%)	31.9 MiB (-2%)	152 (+63%)
relayed-tcp-server2client	29.1 MiB (-2%)	32.0 MiB (-1%)	228 (+55%)

UDP

Test Name	Total/s	Jitter	Lost
direct-udp-client2server	50.0 MiB (-0%)	0.19ms (+7%)	0.00% (NaN%)
direct-udp-server2client	50.0 MiB (-0%)	0.21ms (+29%)	0.00% (NaN%)
relayed-udp-client2server	50.0 MiB (-0%)	0.37ms (+44%)	20.42% (+11%)
relayed-udp-server2client	50.0 MiB (+0%)	0.29ms (+30%)	20.48% (+4%)

[ReactorScram]

The code change looks okay mechanically but I don't understand how making one of these settings strings blank would cause both to be set? Or was that always happening even if only one was blank?

[jamilbk]

I don't understand how making one of these settings strings blank would cause both to be set? Or was that always happening even if only one was blank?

What was happening was that an empty string was causing the addAllowedApplication and addDisallowedApplication functions to be called with an empty string.

Android doesn't let you call both on a tunnel, so it crashed.

The reason both were empty was because when you provision the app with MDM, it sets them to an empty string (there is no null case)