correctly serialize nested sys expressions

of type string
fix possible NPEs in sys object and object array expressions
Merge pull request #1072 from crate/m/string_sys_expressions

correctly serialize nested sys expressions
fix: using the is not null predicate on a non-existing or ignored column
did throw an exception
added tutorial section to docs
wip
tutorial wip
wip
tutorial wip
summary
fixup! summary
fixup! fixup! summary