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/profilechangelistener.h

@@ -27,7 +27,7 @@ class ProfileChangeListener
 
 public:
     virtual     ~ProfileChangeListener() {}
-    virtual void profileChange  (const rtengine::procparams::PartialProfile* nparams, rtengine::ProcEvent event, const Glib::ustring& descr, const ParamsEdited* paramsEdited = NULL) {}
+    virtual void profileChange  (const rtengine::procparams::PartialProfile* nparams, rtengine::ProcEvent event, const Glib::ustring& descr, const ParamsEdited* paramsEdited = nullptr) {}
     virtual void setDefaults    (rtengine::procparams::ProcParams* defparams) {}
 };