Fix FilePath::absoluteFilePath for default constructed file path

FilePath::absoluteFilePath() would consider an empty path to be
relative, and resolved it wrt the current working directory. That is
unexpected in the sense that QFileInfo::absoluteFilePath behaves
differently, and lead to a crash down the line when e.g. a diff editor
was opened when the current working directory is '/' (the default on
macOS when started with "open" or from Finder, Dock or Spotlight).

Make FilePath::absoluteFilePath() return and empty path if the input was
empty.

Change-Id: Ie0d4da50afa24134bd56505b2f4abcf382eee982
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/filepath.cpp

@@ -721,7 +721,11 @@ FilePath FilePath::parentDir() const
 
 FilePath FilePath::absolutePath() const
 {
-    const FilePath parentPath = isAbsolutePath() ? parentDir() : FilePath::currentWorkingPath().resolvePath(*this).parentDir();
+    if (!needsDevice() && isEmpty())
+        return *this;
+    const FilePath parentPath = isAbsolutePath()
+                                    ? parentDir()
+                                    : FilePath::currentWorkingPath().resolvePath(*this).parentDir();
     return parentPath.isEmpty() ? *this : parentPath;
 }
 
@@ -729,6 +733,8 @@ FilePath FilePath::absoluteFilePath() const
 {
     if (isAbsolutePath())
         return *this;
+    if (!needsDevice() && isEmpty())
+        return *this;
 
     return FilePath::currentWorkingPath().resolvePath(*this);
 }