libstdc++: Fix range access for empty std::valarray [PR103022]

libstdc++: Fix range access for empty std::valarray [PR103022]

The std::begin and std::end overloads for std::valarray are defined in
terms of std::addressof(v[0]) which is undefined for an empty valarray.

libstdc++-v3/ChangeLog:

	PR libstdc++/103022
	* include/std/valarray (begin, end): Do not dereference an empty
	valarray. Add noexcept and [[nodiscard]].
	* testsuite/26_numerics/valarray/range_access.cc: Check empty
	valarray. Check iterator properties. Run as well as compiling.
	* testsuite/26_numerics/valarray/range_access2.cc: Likewise.
	* testsuite/26_numerics/valarray/103022.cc: New test.