Fix possible null pointer dereference

Fixes:

*** CID 278346: Null pointer dereferences (NULL_RETURNS)
/home/kurthindenburg/Devel/KDE/src/konsole/src/settings/ConfigurationDialog.h:
124 in Konsole::ConfigDialogButtonGroupManager::hasChanged() const()
118 119 bool hasChanged() const {
120 for(const QButtonGroup *group: qAsConst(_groups)) {
121 int value = buttonToEnumValue(group->checkedButton());
122 const auto enumItem = groupToConfigItemEnum(group);
123
>>> CID 278346: Null pointer dereferences (NULL_RETURNS)
>>> Dereferencing a pointer that might be "nullptr" "enumItem" when
>>> calling "isEqual". (The dereference happens because this is a
>>> virtual function call.)
124 if(!enumItem->isEqual(value)) {
125 return true;
126 }
127 }
128 return false;
129 }