FileName::exists(): Check for empty files first

Fixes warnings
"Empty filename passed to function"
(introduced by qtbase  0dee566e98f5ff4f224e596de1c04de4f9685df4)
emitted when running Qt Creator against 5.10.
The warnings originate from Kit::icon().

Change-Id: I501914195b8e4d340fd02362b215ffc6e3dc99d4
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/utils/fileutils.cpp

@@ -615,7 +615,7 @@ QString FileName::fileName(int pathComponents) const
 /// FileName exists.
 bool FileName::exists() const
 {
-    return QFileInfo::exists(*this);
+    return !isEmpty() && QFileInfo::exists(*this);
 }
 
 /// Find the parent directory of a given directory.