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 {} \;`

rtgui/batchqueueentry.h

@@ -54,7 +54,7 @@ public:
     SaveFormat saveFormat;
     bool forceFormatOpts;
 
-    BatchQueueEntry (rtengine::ProcessingJob* job, const rtengine::procparams::ProcParams& pparams, Glib::ustring fname, int prevw, int prevh, Thumbnail* thm = NULL);
+    BatchQueueEntry (rtengine::ProcessingJob* job, const rtengine::procparams::ProcParams& pparams, Glib::ustring fname, int prevw, int prevh, Thumbnail* thm = nullptr);
     ~BatchQueueEntry ();
 
     void refreshThumbnailImage ();