don't allow null values in ConcurrentArrayList

because that can be the only reason for the CI failure I saw:

ERROR: FAIL java.lang.NullPointerException java.lang.NullPointerException ------------------------------
java.lang.NullPointerException
	at org.h2.mvstore.MVMap.openVersion(MVMap.java:1095)
	at org.h2.test.store.TestConcurrent.testConcurrentChangeAndGetVersion(TestConcurrent.java:346)
	at org.h2.test.store.TestConcurrent.test(TestConcurrent.java:56)
	at org.h2.test.TestBase.runTest(TestBase.java:142)
	at org.h2.test.TestAll.addTest(TestAll.java:990)
	at org.h2.test.TestAll.testUnit(TestAll.java:960)
	at org.h2.test.TestAll.runTests(TestAll.java:625)
	at org.h2.test.TestAll.testAll(TestAll.java:563)
	at org.h2.test.TestAll.run(TestAll.java:512)
	at org.h2.test.TestAll.main(TestAll.java:452)

Assorted changes in tests and documentation

Check precision for FLOAT data type and describe it in documentation

Add support for MySQL-style ALTER TABLE ADD ... FIRST

Derived column list

remove dead code

Nest joins only if required and fix some issues with complex joins

found by UCDetector

Fix obscure error on non-standard SELECT * FROM A LEFT JOIN B NATURAL JOIN C

Move some JOIN tests from testScript.sql to own file