Address https://bugs.kde.org/show_bug.cgi?id=56184#c94

> The list was missing the error
> " Unsupported xmi file version: "
>
> Notice that after "version:" there is no version indication.

umbrello/umldoc.cpp
- In function loadFromXMI1, if both "xmi.version" and "xmi:version" are
  not found then do not assume versionString.toDouble() as the version
  because in this case, versionString is empty.
  In the interest of best effort, attempt loading the file if the XMI
  version is unknown.

CCBUG: 56184