fix: provide error instead of nil panic when cache_zone is missing

[Revolyssup]

Description

Fixes #9957

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)

[monkeyDluffy6017]

It's not very reliable to judge by cache zone name, i think you could modified like this:

if local_conf.apisix.proxy_cache then
    for _, cache in ipairs(local_conf.apisix.proxy_cache.zones) do
        if cache.name == conf.cache_zone then
            if conf.cache_strategy == STRATEGY_MEMORY and not cache.disk_path then
                found = true
                break
            elseif conf.cache_strategy == STRATEGY_DISK and cache.disk_path then
                found = true
                break
            end
        end
    end
    if found == false then
        return false, "cache_zone " .. conf.cache_zone .. " not found"
    end
end

[Revolyssup]

@monkeyDluffy6017 I added a slightly modified version of your suggestion with comments. Please review.