fprintf nullptr checks

Simple fix to avoid "desktoptojson: vfprintf %s NULL in "Warning: %s(%s:%u, %s)" warnings in OpenBSD. OpenBSD's vfprintf impl. warns if %s gets NULL. The template lambda function need c++14 so I adapted the CMake part to be sure we compile this with C++14 enabled.

Env

System

OpenBSD 6.7-current (GENERIC.MP) #36: Sat Aug 22 11:27:03 MDT 2020
    deraadt@amd64.openbsd.org:/usr/src/sys/arch/amd64/compile/GENERIC.MP
real mem = 17023250432 (16234MB)
avail mem = 16492290048 (15728MB)

Compiler

clang --version
OpenBSD clang version 10.0.1
Target: amd64-unknown-openbsd6.7
Thread model: posix
InstalledDir: /usr/bin

Qt

qmake-qt5 -v
QMake version 3.1
Using Qt version 5.13.2 in /usr/local/lib/qt5

CMake

cmake --version
cmake version 3.17.2

CMake suite maintained and supported by Kitware (kitware.com/cmake).

Error / Issue

from syslog:

desktoptojson: vfprintf %s NULL in "Warning: %s (%s:%u, %s) "1

Background Information

I am working on the KDE/Qt packages for OpenBSD. I am an active maintainer of CMake, Qt and KDE Frameworks and KDE Applications on OpenBSD. I want to help with this PR to achieve better support for KDE on OpenBSD.