Which One Is Standard?
If you want to use 'nothing' as an indefinite pronoun, always write it as one single word.
'No thing' is not a pronoun, but a noun phrase. Let's take a closer look at each one:
When we put 'no' and 'thing' as a two-word noun phrase, i.e. 'no thing', note that the 'thing' must be a concrete thing (a thing that can be seen or touched), so when we say 'no thing', it means there's a presence there, but it has no touchable form.
I thought I saw something there, but it was
Another use of the noun phrase 'no thing' is when we want to put another word between the two, for example:
'Nothing' is an indefinite pronoun. It means 'not anything'. It shows the lack of a single thing.
Don't Use Negative Verbs with 'Nothing'
Always keep in mind that we do NOT use negative verbs with the pronoun 'nothing'. Because 'nothing' is a negative marker by itself.
This is a case of 'double negation' which is not grammatically correct.