1. No need to call `hashCode` if  `cache=null`
2. Remove redundant `null` comparison. `String.equals` will anyway return `false`

Added test case

convert ((A=1 AND B=2) OR (A=1 AND B=3)) to (A=1 AND (B=2 OR B=3))
-Suggested changes in:
https://github.com/h2database/h2database/pull/1571#discussion_r235925138
https://github.com/h2database/h2database/pull/1571#discussion_r235925223

convert ((A=1 AND B=2) OR (A=1 AND B=3)) to (A=1 AND (B=2 OR B=3))