Allow empty names for attached databases

An empty database name is not the same as pressing the cancel button
when attaching a database.

See issue #1132.

src/sqlitedb.cpp

@@ -168,7 +168,7 @@ bool DBBrowserDB::attach(const QString& filename, QString attach_as)
                                           QLineEdit::Normal,
                                           QFileInfo(filename).baseName()
                                           ).trimmed();
-    if(attach_as.isEmpty())
+    if(attach_as.isNull())
         return false;
 
 #ifdef ENABLE_SQLCIPHER