In this implementation it is an error to pass the empty path to absolute,
because the empty path doesn't represent any file in the filesystem so
the function cannot meet its postcondition.

Currently the absolute(const path&, error_code&) overload reports an
error for the empty path, but using errc::no_such_file_or_directory, and
the other overload does not report an error. This patch makes them
consistntly report an errc::invalid_argument error for the empty path.

	PR libstdc++/90299
	* src/c++17/fs_ops.cc (absolute(const path&)): Report an error if the
	argument is an empty path.
	(absolute(const path&, error_code&)): Use invalid_argument as error
	code instead of no_such_file_or_directory.
	* testsuite/27_io/filesystem/operations/absolute.cc: Check handling
	of non-existent paths and empty paths with both overloads of absolute.

From-SVN: r270874