Throw DbException if database encryption key is missing.

This can happen on Android if the app is automatically restarted after a
crash.

briar-core/src/net/sf/briar/db/H2Database.java

@@ -78,7 +78,7 @@ class H2Database extends JdbcDatabase {
 
 	protected Connection createConnection() throws SQLException {
 		byte[] key = config.getEncryptionKey();
-		if(key == null) throw new IllegalStateException();
+		if(key == null) return null;
 		char[] password = encodePassword(key);
 		Properties props = new Properties();
 		props.setProperty("user", "user");