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/partialpastedlg.h

@@ -134,7 +134,7 @@ public:
 public:
     explicit PartialPasteDlg (const Glib::ustring &title);
 
-    void applyPaste (rtengine::procparams::ProcParams* dstPP, ParamsEdited* dstPE, const rtengine::procparams::ProcParams* srcPP, const ParamsEdited* srcPE = NULL);
+    void applyPaste (rtengine::procparams::ProcParams* dstPP, ParamsEdited* dstPE, const rtengine::procparams::ProcParams* srcPP, const ParamsEdited* srcPE = nullptr);
 
     void everythingToggled ();
     void basicToggled ();