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/coloredbar.cc

@@ -201,5 +201,5 @@ void ColoredBar::clearBgGradient ()
 
 bool ColoredBar::canGetColors()
 {
-    return colorProvider != NULL || bgGradient.size() > 0;
+    return colorProvider != nullptr || bgGradient.size() > 0;
 }