vpn: Fix NPE in VpnConfig#toString when no ProxyInfo is set

Bug: 150910582
Test: Compiled bertha_x86_64-userdebug (Chrome OS ARCVM image) for
rvc-dev, flashed eve-arcvm (Pixelbook), connected TouchVpn App, observed
that ARC's VpnTracker does not crash the system server when printing
VpnConfig objects with no ProxyInfo.

Change-Id: I3f8dd1d232e2b18dae9e99eb05c92a797a2358b9