Don't paste NULL value when pasting from empty clipboard

When the clipboard is empty and trying to paste from it we would set the
selected field to NULL. This is fixed by this commit, so that nothing
happens.

src/ExtendedTableWidget.cpp

@@ -32,8 +32,11 @@ QList<QStringList> parseClipboard(QString clipboard)
     if(clipboard.endsWith("\r"))
         clipboard.chop(1);
 
-    // Prepare result list
+    // Make sure there is some data in the clipboard
     QList<QStringList> result;
+    if(clipboard.isEmpty())
+        return result;
+
     result.push_back(QStringList());
 
     QRegExp re("(\"(?:[^\t\"]+|\"\"[^\"]*\"\")*)\"|(\t|\r?\n)");