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/rtengine.h

@@ -176,7 +176,7 @@ public:
       * @param errorCode is a pointer to a variable that is set to nonzero if an error happened (output)
       * @param pl is a pointer pointing to an object implementing a progress listener. It can be NULL, in this case progress is not reported.
       * @return an object representing the loaded and pre-processed image */
-    static InitialImage* load (const Glib::ustring& fname, bool isRaw, int* errorCode, ProgressListener* pl = NULL);
+    static InitialImage* load (const Glib::ustring& fname, bool isRaw, int* errorCode, ProgressListener* pl = nullptr);
 };
 
 /** When the preview image is ready for display during staged processing (thus the changes have been updated),
@@ -482,7 +482,7 @@ public:
    * @param pl is an optional ProgressListener if you want to keep track of the progress
    * @param tunnelMetaData tunnels IPTC and XMP to output without change
    * @return the resulting image, with the output profile applied, exif and iptc data set. You have to save it or you can access the pixel data directly.  */
-IImage16* processImage (ProcessingJob* job, int& errorCode, ProgressListener* pl = NULL, bool tunnelMetaData = false, bool flush = false);
+IImage16* processImage (ProcessingJob* job, int& errorCode, ProgressListener* pl = nullptr, bool tunnelMetaData = false, bool flush = false);
 
 /** This class is used to control the batch processing. The class implementing this interface will be called when the full processing of an
    * image is ready and the next job to process is needed. */