fix: check if table entry is nil or not (#1867)

running 

```luarocks config local_by_default``` returns ```Error: Unknown entry local_by_default```

and running

```luarocks config local_by_default true``` writes it as a string to the config file: 
```lua 
local_by_default = "true"
```

Same is true to any bool var set as false.

This pr aims to fix this issue.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/luarocks/cmd/config.lua b/src/luarocks/cmd/config.lua
index e8bda657..c350e42a 100644
--- a/src/luarocks/cmd/config.lua
+++ b/src/luarocks/cmd/config.lua
@@ -130,7 +130,7 @@ end
 
 local function print_entry(var, tbl, is_json)
    return traverse_varstring(var, tbl, function(t, k)
-      if not t[k] then
+      if t[k] == nil then
          return nil, "Unknown entry " .. k
       end
       local val = t[k]
@@ -151,7 +151,7 @@ end
 local function infer_type(var)
    local typ
    traverse_varstring(var, cfg, function(t, k)
-      if t[k] then
+      if t[k] ~= nil then
          typ = type(t[k])
       end
    end)