caps: Don't assert in fixate() on EMPTY/ANY caps and document EMPTY/ANY behaviour on more functions
fixate() will return empty caps if it gets empty caps passed and assert
early if any caps are provided as there's no meaningful way of fixating
any caps.
truncate() and simplify() will return the input caps in case of
any/empty caps as before, but slightly optimized and as documented
behaviour.
Also add tests for this and a few other operations behaviour on
empty/any caps.
Edited by Sebastian Dröge