Ninja: Restore support for compilers not defining a C++ standard level

Since commit 386465bf83 (cmTarget: add support for C++ module fileset
types, 2022-04-08, v3.25.0-rc1~624^2~7), the Ninja generator checks for
C++20 support using logic that requires `CMAKE_<LANG>_STANDARD_DEFAULT`
to be non-empty.  On some compilers, such as ARMClang, CMake does not
automatically detect and set default language standards, thus causing
`HaveStandardAvailable` to raise an internal error.

To fix this issue, if `CMAKE_CXX_STANDARD_DEFAULT` is empty, assume all
standards to be supported instead of calling `HaveStandardAvailable`.
This is consistent with how `CompileFeaturesNode::Evaluate` handles this
case.

Fixes: #24146

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