fix(traffic-split): `upstream_obj.upstream` should not be a string (#… #17292

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	34m 21s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	36m 56s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	28m 48s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	28m 9s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	33m 53s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	34m 39s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	26m 58s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	27m 19s
	4h 11m 3s

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