Fix: YAML: force enumeration FT objects to have at least one mapping

Fix: YAML: force enumeration FT objects to have at least one mapping

As per CTF 1.8.3:

> Enumerations need to contain at least one entry.

Before this patch, ythose were legal:

    class: uenum
    size: 8

    class: uenum
    size: 8
    mappings: {}

    class: uenum
    size: 8
    mappings:
      A: []

CTF consumers would not like the resulting metadata stream. For
example, Babeltrace 2.1.0-rc1 [v1.2.0-3520-g73f74ae1f]:

    09-24 13:26:41.740 80492 80492 E PLUGIN/CTF/META/PARSER yyerror@parser.y:925 At line 109 in metadata stream: syntax error, unexpected CTF_RBRAC: token="}"
    09-24 13:26:41.740 80492 80492 E PLUGIN/CTF/META/DECODER ctf_metadata_decoder_append_content@decoder.c:280 [N/A] Cannot create the metadata AST out of the metadata text: mdec-addr=0x559dd9546a10
    09-24 13:26:41.740 80492 80492 W PLUGIN/SRC.CTF.FS/QUERY support_info_query@query.c:459 cannot append metadata content: metadata-decoder-status=-2
    09-24 13:26:41.741 80492 80492 W LIB/QUERY-EXECUTOR bt_query_executor_query@query-executor.c:231 Component class's "query" method failed: query-exec-addr=0x559dd9548070, cc-addr=0x559dd95233e0, cc-type=SOURCE, cc-name="fs", cc-partial-descr="Read CTF traces from the file sy", cc-is-frozen=0, cc-so-handle-addr=0x559dd9522770, cc-so-handle-path="lib/babeltrace2/plugins/babeltrace-plugin-ctf.so", object="babeltrace.support-info", params-addr=0x559dd9545f10, params-type=MAP, params-element-count=2, log-level=WARNING
    09-24 13:26:41.741 80492 80492 E CLI-CFG-SRC-AUTO-DISC support_info_query_all_sources@autodisc.c:493 babeltrace.support-info query failed.
    09-24 13:26:41.741 80492 80492 E CLI main@babeltrace2.c:2644 Command-line error: retcode=1

    ERROR:    [Babeltrace CLI] (babeltrace2.c:2644)
      Command-line error: retcode=1
    CAUSED BY [Source auto-discovery] (autodisc.c:493)
      babeltrace.support-info query failed.
    CAUSED BY [libbabeltrace2] (query-executor.c:231)
      Component class's "query" method failed: query-exec-addr=0x559dd9548070, cc-addr=0x559dd95233e0, cc-type=SOURCE, cc-name="fs", cc-partial-descr="Read CTF traces from
      the file sy", cc-is-frozen=0, cc-so-handle-addr=0x559dd9522770, cc-so-handle-path="babeltrace2/plugins/babeltrace-plugin-ctf.so",
      object="babeltrace.support-info", params-addr=0x559dd9545f10, params-type=MAP, params-element-count=2, log-level=WARNING

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>