fix(read-configs): json decode + nil schema

[gnikolaos]

In 2 different projects where I use nx.nvim I met the below errors:

With the first one, I was getting the following error every time I opened nvim: -> attempt to concatenate field 'schema' (a nil value) in 'callback'. This commit adds a check on the schema value and resolves the issue.

At the 2nd one, I was getting the following errors popping up everytime I was opening nvim: -> expected object key string but found T_OBJ_END at character ... in 'decode'. -> expected object value but found T_ARR_END at character ... in 'decode'. After ignoring them, my generators and actions list was empty. This commit adds a protected call, and the actions and generators lists are now populated, with no errors popping up.