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/pparamschangelistener.h

@@ -28,7 +28,7 @@ class PParamsChangeListener
 
 public:
     virtual     ~PParamsChangeListener() {}
-    virtual void procParamsChanged (rtengine::procparams::ProcParams* params, rtengine::ProcEvent ev, Glib::ustring descr, ParamsEdited* paramsEdited = NULL) {}
+    virtual void procParamsChanged (rtengine::procparams::ProcParams* params, rtengine::ProcEvent ev, Glib::ustring descr, ParamsEdited* paramsEdited = nullptr) {}
     virtual void clearParamChanges () {}
 };