bug: proxy-cache plugin generate_complex_value may fail due to concat nil values

[worldwonderer]

Current Behavior

if proxy-cache plugins configures cache_key with ["$arg_a", "$arg_b"] and $arg_a and $arg_b are both nil, apisix will abort with error.

I think it is quite normal that api has not required args, can this line tmp[i] = ctx.var[string.sub(value, 2)] change to tmp[i] = ctx.var[string.sub(value, 2)] or ""? if it is acceptable, i would like to submit a PR.

Expected Behavior

cache_key not fail with nil variables

Error Logs

[error] 3329653#3329653: *15666726 lua entry thread aborted: runtime error: ...tion/bin/apisix/plugins/proxy-cache.lua:149: invalid value (nil) at index 2 in table for 'concat'

Steps to Reproduce

create route with proxy-cache plugin, and configure cache key with "cache_key": ["$arg_a", "$arg_b"] and then request without url args

Environment

• APISIX version (run apisix version):2.11.0
• Operating system (run uname -a):Linux 11-186-252-137 4.14.105-1-tlinux3-0016 change: added doc of how to load plugin. #1 SMP Fri Jul 31 16:16:47 CST 2020 x86_64 x86_64 x86_64 GNU/Linux
• OpenResty / Nginx version (run openresty -V or nginx -V):nginx version: openresty/1.19.9.1

[soulbird]

Welcome to contribute PR！