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

core/java/com/android/internal/net/VpnConfig.java

@@ -232,7 +232,7 @@ public class VpnConfig implements Parcelable {
                 .append(", allowIPv4=").append(allowIPv4)
                 .append(", allowIPv6=").append(allowIPv6)
                 .append(", underlyingNetworks=").append(Arrays.toString(underlyingNetworks))
-                .append(", proxyInfo=").append(proxyInfo.toString())
+                .append(", proxyInfo=").append(proxyInfo)
                 .append("}")
                 .toString();
     }