Apply modernize-use-nullptr

Setup:
- `mkdir tidy; cd tidy`
- `cmake .. -DCMAKE_BUILD_TYPE=debug -DPROC_TARGET_NUMBER=1 -DCACHE_NAME_SUFFIX=4 -DBINDIR=. -DDATADIR=. -DBUILD_BUNDLE=ON -DWITH_LTO=OFF -DOPTION_OMP=OFF -DCMAKE_EXPORT_COMPILE_COMMANDS=ON`
- `cd ..`
- `find -name '*.cc' -exec clang-tidy-3.8 -header-filter=.* -p=tidy -fix-errors -checks=modernize-use-nullptr {} \;`

rtengine/processingjob.h

@@ -34,7 +34,7 @@ public:
     procparams::ProcParams pparams;
 
     ProcessingJobImpl (const Glib::ustring& fn, bool iR, const procparams::ProcParams& pp)
-        : fname(fn), isRaw(iR), initialImage(NULL), pparams(pp) {}
+        : fname(fn), isRaw(iR), initialImage(nullptr), pparams(pp) {}
 
     ProcessingJobImpl (InitialImage* iImage, const procparams::ProcParams& pp)
         : fname(""), initialImage(iImage), pparams(pp)