Drop a C99/C++20 struct initializer and fix typo.

The aggregrated initialization with a designator is standardized
only in C++20 for C++, and was causing build failures for OpenSUSE
leap. Simply abstain from initializing uid as the default value of ""
should be sufficient.

* src/conversationsview.cpp (render_contact_photo): Rename
invalidConverstation to invalidConversation.  Do not explicitly
initialize the 'uid' field.
* src/mainwindow.cpp (CppImpl::getCurrentConversation): Adjust accordingly.

Change-Id: I0a6731910750f30523c2f956fecf5d7e97bceb3a