fix: PoC fix for #11781, "connect to apisix failed: missing the port number"

[ShaunMaher]

Description

Fixes #11781

In the event that ngx.var.server_port is a blank string (e.g. client request arrived via unix socket instead of TCP port), httpc:connect is given the listening port from the APISIX config, preventing an error.

I have tried to cover the possible ways that node_port can be defined in the config.

function tab_is_array(t) borrowed from:

apisix/apisix/cli/file.lua

Line 47 in a0b1a45

local function tab_is_array(t)

Sorry if my Lua code isn't up to your standards, I'm new to Lua and still finding my feet.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change: I'll need help with this
• I have updated the documentation to reflect this change: I don't think any changes are necessary.
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)