MergeError if `tag` as list inherits `tag` as a string from the upper node

[smyle10]

tmt version: 1.35.0

Given a main.fmf:

test: TEST_ITEMS=${TMT_TEST_NAME##*/} bash ./runtest.sh
tag: foobar
/test_keep_iproute_routes:
    tag+:
      - customer
      - unmanaged

Running tmt test ls got:

MergeError: Key 'tag' in /networking/nmstate/anyhost/route/test_keep_iproute_routes (can only concatenate str (not "list") to str).

Though tmt hints the reason and I can change string "foobar" as a list (but consider a new tmt user may not know the inheritance feature in tmt) , as https://tmt.readthedocs.io/en/stable/spec/core.html#tag indicates that a tag can be string or list, hope such merge can be improved.