libstdc++: Fix std::format for localized floats [PR110968]

The __formatter_fp::_M_localize function just returns an empty string if
the formatting locale is the C locale, as there is nothing to do. But
the caller was assuming that the returned string contains the localized
string. The caller should use the original string if _M_localize returns
an empty string.

libstdc++-v3/ChangeLog:

	PR libstdc++/110968
	* include/std/format (__formatter_fp::format): Check return
	value of _M_localize.
	* testsuite/std/format/functions/format.cc: Check classic
	locale.