Current Behavior

when I try to conf apisix.yaml with ssls,like:
ssls:

cert: |
     -----BEGIN CERTIFICATE-----
     xxxxxxxxxxxxxxxxxxxxxxxxxx
     -----END CERTIFICATE-----
key: |
    -----BEGIN PRIVATE KEY-----
     xxxxxxxxxxxxxxxxxxxxxxxxxx
     -----END PRIVATE KEY-----
certs:
  - |
     -----BEGIN CERTIFICATE-----
     xxxxxxxxxxxxxxxxxxxxxxxxxx
     -----END CERTIFICATE-----
keys:
  - |
     -----BEGIN PRIVATE KEY-----
     xxxxxxxxxxxxxxxxxxxxxxxxxx
     -----END PRIVATE KEY-----
snis:
  - "localhost"

#END

Expected Behavior

this is a yaml parsing, maybe this error shoud not happen?

Error Logs

/usr/local//luajit/bin/luajit: /usr/local/apisix/deps/share/lua/5.1/tinyyaml.lua:345: attempt to compare nil with number
stack traceback:
/usr/local/apisix/deps/share/lua/5.1/tinyyaml.lua:345: in function 'parsescalar'
/usr/local/apisix/deps/share/lua/5.1/tinyyaml.lua:606: in function 'parseseq'
/usr/local/apisix/deps/share/lua/5.1/tinyyaml.lua:739: in function 'parsemap'
/usr/local/apisix/deps/share/lua/5.1/tinyyaml.lua:597: in function 'parseseq'
/usr/local/apisix/deps/share/lua/5.1/tinyyaml.lua:739: in function 'parsemap'
/usr/local/apisix/deps/share/lua/5.1/tinyyaml.lua:801: in function 'parsedocuments'
/usr/local/apisix/deps/share/lua/5.1/tinyyaml.lua:825: in function 'parse'
/usr/local/apisix/apisix/cli/file.lua:293: in function 'read_yaml_conf'
/usr/local/apisix/apisix/cli/ops.lua:174: in function 'init'
/usr/local/apisix/apisix/cli/ops.lua:840: in function </usr/local/apisix/apisix/cli/ops.lua:756>
/usr/local/apisix/apisix/cli/ops.lua:973: in function 'execute'
/usr/local/apisix/apisix/cli/apisix.lua:40: in main chunk
[C]: at 0x004051e0

Steps to Reproduce

1.write conf to apisix.yaml
2. apisix start

Environment

• APISIX version (run apisix version):3.4.1
• Operating system (run uname -a):Linux ubuntu 4.15.0-142-generic perf: removed useless split uri for admin APIs. #146~16.04.1-Ubuntu SMP Tue Apr 13 09:27:15 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
• OpenResty / Nginx version (run openresty -V or nginx -V):nginx version: openresty/1.21.4.1
• etcd version, if relevant (run curl http://127.0.0.1:9090/v1/server_info):"etcd_version":"3.5.0"
• APISIX Dashboard version, if relevant:
• Plugin runner version, for issues related to plugin runners:
• LuaRocks version, for installation issues (run luarocks --version):luarocks 3.9.2