Fix gcc 11 -Wnonnull warnings

[rouault]

/proj-8.0.0/src/iso19111/operation/coordinateoperationfactory.cpp: In function 'osgeo::proj::operation::TransformationNNPtr osgeo::proj::operation::createBallparkGeographicOffset(const CRSNNPtr&, const CRSNNPtr&, const DatabaseContextPtr&)':
/proj-8.0.0/src/iso19111/operation/coordinateoperationfactory.cpp:1860:36: warning: 'this' pointer is null [-Wnonnull]
 1860 |                 ->coordinateSystem()
      |                                    ^
In file included from /proj-8.0.0/src/iso19111/operation/coordinateoperationfactory.cpp:35:
/proj-8.0.0/include/proj/crs.hpp:196:47: note: in a call to non-static member function 'const CoordinateSystemNNPtr& osgeo::proj::crs::SingleCRS::coordinateSystem() const'
  196 |     PROJ_DLL const cs::CoordinateSystemNNPtr &coordinateSystem() PROJ_PURE_DECL;
      |                                               ^~~~~~~~~~~~~~~~
/proj-8.0.0/src/iso19111/operation/coordinateoperationfactory.cpp:1864:36: warning: 'this' pointer is null [-Wnonnull]
 1864 |                 ->coordinateSystem()
      |                                    ^
In file included from /proj-8.0.0/src/iso19111/operation/coordinateoperationfactory.cpp:35:
/proj-8.0.0/include/proj/crs.hpp:196:47: note: in a call to non-static member function 'const CoordinateSystemNNPtr& osgeo::proj::crs::SingleCRS::coordinateSystem() const'
  196 |     PROJ_DLL const cs::CoordinateSystemNNPtr &coordinateSystem() PROJ_PURE_DECL;
      |                                               ^~~~~~~~~~~~~~~~
/proj-8.0.0/src/iso19111/factory.cpp: In member function 'std::vector<dropbox::oxygen::nn<std::shared_ptr<osgeo::proj::operation::CoordinateOperation> > > osgeo::proj::io::AuthorityFactory::createBetweenGeodeticCRSWithDatumBasedIntermediates(const CRSNNPtr&, const string&, const string&, const CRSNNPtr&, const string&, const string&, bool, bool, bool, bool, const std::vector<std::__cxx11::basic_string<char> >&, const ExtentPtr&, const ExtentPtr&) const':
/proj-8.0.0/src/iso19111/factory.cpp:4724:66: warning: 'this' pointer is null [-Wnonnull]
 4724 |         dynamic_cast<crs::GeodeticCRS *>(sourceCRS.get())->datum();
      |                                                                  ^
In file included from /proj-8.0.0/src/iso19111/factory.cpp:36:
/proj-8.0.0/include/proj/crs.hpp:254:54: note: in a call to non-static member function 'const GeodeticReferenceFramePtr& osgeo::proj::crs::GeodeticCRS::datum() const'
  254 |     PROJ_DLL const datum::GeodeticReferenceFramePtr &datum() PROJ_PURE_DECL;
      |                                                      ^~~~~
/proj-8.0.0/src/iso19111/factory.cpp:4726:66: warning: 'this' pointer is null [-Wnonnull]
 4726 |         dynamic_cast<crs::GeodeticCRS *>(targetCRS.get())->datum();
      |                                                                  ^
In file included from /proj-8.0.0/src/iso19111/factory.cpp:36:
/proj-8.0.0/include/proj/crs.hpp:254:54: note: in a call to non-static member function 'const GeodeticReferenceFramePtr& osgeo::proj::crs::GeodeticCRS::datum() const'
  254 |     PROJ_DLL const datum::GeodeticReferenceFramePtr &datum() PROJ_PURE_DECL;
      |                                                      ^~~~~

[rouault]

Manually cherry-picked to 8.0 branch