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, 38 insertions, 0 deletions

diff --git a/spec/quick/config.q b/spec/quick/config.q
index d6230567..dd2dcd00 100644
--- a/spec/quick/config.q
+++ b/spec/quick/config.q
@@ -95,3 +95,41 @@ Warning: Failed finding Lua header lua.h (searched at /some/bad/path). You may n
 
 LuaRocks may not work correctly when building C modules using this configuration.
 --------------------------------------------------------------------------------
+
+
+
+================================================================================
+TEST: reports when getting a falsy boolean variable
+
+RUN: luarocks config local_by_default
+
+STDOUT:
+--------------------------------------------------------------------------------
+false
+--------------------------------------------------------------------------------
+
+
+
+================================================================================
+TEST: reports when setting a falsy boolean variable
+
+RUN: luarocks config local_by_default true
+
+STDOUT:
+--------------------------------------------------------------------------------
+Wrote
+local_by_default = true
+--------------------------------------------------------------------------------
+
+
+
+================================================================================
+TEST: reports when getting an unknown variable
+
+RUN: luarocks config foo
+EXIT: 1
+
+STDERR:
+--------------------------------------------------------------------------------
+Error: Unknown entry foo
+--------------------------------------------------------------------------------