TypeSystem: There should be explicitly written that composite/array types accepts NULLs everywhere

The following discussion from !40 (merged) should be addressed:

• @jkuchar1, @jpudil started a discussion: (+1 comment)

  There is implicit support for nullable types, shouldn't it be explicit instead?

mentioned in commit ac84cb0e

mentioned in commit 0ad3e500

mentioned in commit 618aa4a2

mentioned in commit e7399fd4

mentioned in merge request !58 (merged)

mentioned in commit f721a127

mentioned in commit 82442ffa

mentioned in commit 11e28966

closed with commit 82442ffa

closed with merge request !58 (merged)