fix: do not invoke add_header if value resolved as nil

[fengxsong]

Description

If header_value is resolved to nil or empty table, add_header will throw an error https://github.com/openresty/lua-resty-core/blob/master/lib/resty/core/request.lua#L364

It's better to do nothing.

ref: https://github.com/openresty/lua-resty-core/blob/master/lib/ngx/req.md#add_header

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 have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[Revolyssup]

@fengxsong Can you add some test here?

[fengxsong]

@fengxsong Can you add some test here?

do we really need test case for this PR?

[Revolyssup]

@fengxsong Can you add some test here?

do we really need test case for this PR?

A test case proving that logs have no error log in this condition might be helpful. @Sn0rt Do you have suggestions here?

[monkeyDluffy6017]

@Revolyssup I think the code is very simple, no test cases is ok