ObjC: Proper initialization of ObjC/XX standard properties

Fix logic added by commit 81566557 (ObjC: Initialize ObjC/XX standard
properties from C/C++ counterparts, 2019-11-09) to account for cases
when the CXX standard is not explicitly set.  Also, do not copy the
`*_STANDARD_REQUIRED` and `*_EXTENSIONS` properties unless we copied the
`*_STANDARD` property.
Status Name Job ID Coverage
  External
passed cdash

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

00:10:50

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

00:14:15

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

passed cmake-moonshade-osx-release-makefiles

passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager