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/rawimagesource.h

@@ -138,7 +138,7 @@ public:
     void        getImage    (const ColorTemp &ctemp, int tran, Imagefloat* image, const PreviewProps &pp, const ToneCurveParams &hrp, const ColorManagementParams &cmp, const RAWParams &raw);
     eSensorType getSensorType () const
     {
-        return ri != NULL ? ri->getSensorType() : ST_NONE;
+        return ri != nullptr ? ri->getSensorType() : ST_NONE;
     }
     ColorTemp   getWB       () const
     {