Empty transport configs/properties should be treated the same as nulls.

...e/src/net/sf/briar/plugins/bluetooth/BluetoothPlugin.java

...src/net/sf/briar/plugins/droidtooth/DroidtoothPlugin.java

briar-core/src/net/sf/briar/plugins/modem/ModemPlugin.java

...re/src/net/sf/briar/plugins/modem/ModemPluginFactory.java

briar-core/src/net/sf/briar/plugins/tcp/LanTcpPlugin.java

briar-core/src/net/sf/briar/plugins/tcp/TcpPlugin.java

briar-core/src/net/sf/briar/plugins/tcp/WanTcpPlugin.java

briar-core/src/net/sf/briar/plugins/tor/TorPlugin.java

...r-core/src/net/sf/briar/plugins/tor/TorPluginFactory.java

briar-core/src/net/sf/briar/util/StringUtils.java

briar-tests/src/net/sf/briar/plugins/tor/TorPluginTest.java