Refactor: libcrmcommon: pcmk__xml_show() returns standard Pacemaker RC

We use the RC selector function created in a previous commit.

• pcmk_rc_no_output if nothing is output or if all calls to out->info() return pcmk_rc_no_output (for example, quiet mode is enabled or fmt_name == "none")
• pcmk_rc_ok otherwise (expected)
• some other return code in case of an unforeseen error

We also move show_xml_node()'s NULL check to pcmk__xml_show(), so that
we don't allocate a GString in that case. The only other caller,
show_xml_element(), should be unable to pass a NULL data argument.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>