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/imagedata.h

@@ -54,7 +54,7 @@ protected:
 
 public:
 
-    ImageData (Glib::ustring fname, RawMetaDataLocation* rml = NULL);
+    ImageData (Glib::ustring fname, RawMetaDataLocation* rml = nullptr);
     virtual ~ImageData ();
 
     const rtexif::TagDirectory*   getExifData () const