Don't open nonexistent documents from problems view + assertion fix

Summary:
Previous version always call documentController to open current problem item. This leads to open new (empty) documents for items with finalLocation() refers to nonexistent document. Such items produced by some analyzers plugins like Cppcheck or Valgrind.

The patch also fixes assertion from shell/documentcontroller.cpp:218:

Q_ASSERT(!inputUrl.isRelative());

This assertion caused, for example, by problems with empty finalLocation() (displayed as "/" in view).

Test Plan: Tested on master branch

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4642