Import: don't assume an empty file is a text file

Since in any case, the empty file has nothing to import and it's admitted by
SQLite as a valid database, it's better to revert to the previous behaviour,
where an empty file dropped or opened by the file manager was treated as a DB
and indeed worked.

The behaviour was changed by cfa4dee3be

See issue #2636

src/Data.cpp

@@ -65,7 +65,7 @@ bool isTextOnlyFile(const QString& fileName)
     QFile file(fileName);
     if (file.open(QFile::ReadOnly)) {
         QByteArray data = file.read(512);
-        return isTextOnly(data);
+        return !data.isEmpty() && isTextOnly(data);
     } else {
         return false;
     }