[Fix](Variant) fix variant with empty key

[eldenmoon]

in some senario empty key will cause crash like

*** tablet *** SIGSEGV unknown detail explain (@0x0) received by PID 1527747 (
TID 1544788 OR 0x7f3302988700) from PID 0; stack trace: ***
 0# doris::signal::(anonymous namespace)::FailureSignalHandler(int, siginfo_t*
, void*) at /mnt/disk2/lihangyu/doris/be/src/common/signal_handler.h:429
 1# 0x00007F4880A12B50 in /lib64/libc.so.6
 2# doris::vectorized::PathInDataBuilder::append(std::basic_string_view<char,
std::char_traits<char> >, bool) at /mnt/disk2/lihangyu/doris/be/src/vec/json/p
ath_in_data.cpp:193
 3# doris::vectorized::JSONDataParser<doris::vectorized::SimdJSONParser, false
>::traverseObject(doris::vectorized::SimdJSONParser::Object const&, doris::vec
torized::JSONDataParser<doris::vectorized::SimdJSONParser, false>::ParseContex
t&) at /mnt/disk2/lihangyu/doris/be/src/vec/json/json_parser.cpp:121
 4# doris::vectorized::JSONDataParser<doris::vectorized::SimdJSONParser, false
>::traverse(doris::vectorized::SimdJSONParser::Element const&, doris::vectoriz
ed::JSONDataParser<doris::vectorized::SimdJSONParser, false>::ParseContext&) a
t /mnt/disk2/lihangyu/doris/be/src/vec/json/json_parser.cpp:95
 5# doris::vectorized::JSONDataParser<doris::vectorized::SimdJSONParser, false
>::parse(char const*, unsigned long) at /mnt/disk2/lihangyu/doris/be/src/vec/j
son/json_parser.cpp:81

Proposed changes

Issue Number: close #xxx

[doris-robot]

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[github-actions]

clang-tidy review says "All clean, LGTM! 👍"