Simplify draft-07 meta-schema

[epoberezkin]

• remove unnecessary "items": {} in examples
• use boolean schema true as the default value of keywords where the default is {}

I can make a PR

[epoberezkin]

The same problem exists in draft-06 meta-schema, it can be updated as well.

[handrews]

I saw that at some point after pushing the draft-06 meta-schema and decided it was not worth changing given that someone shows up and complains every time we "fix" it. So I'd rather leave draft-06 alone except for actual bugs.

I'd be fine with consistently using true instead of {} in draft-07, though, so if you'd like to submit a PR for that part, please do!

However, I put the technically-unnecessary items in for examples because otherwise someone thought it was an accident that examples didn't specify something. Think of it as documentation emphasizing that the contents of the array are unconstrained.

Alternatively, you could remove it and put in a $comment explaining it, but why write a comment with the schema can show it clearly?

[epoberezkin]

ok, thanks

[handrews]

Merged #495