[CALCITE-6552] Enable CheckerFramework in 'server' module

[julianhyde]

In CALCITE-4199 we enabled CheckerFramework (via annotations and CI jobs) in the 'core' and 'linq4j' modules; this change further enables CheckerFramework in the 'server' module, and fixes all violations.

There are also a large number of 'cosmetic' modifications to improve code quality without changing behavior, including:

• Replace this.x = x; assert x != null; with this.x = requireNonNull(x); in constructors
• Replace assert in other code locations where it implements an invariant. We don't use requireNonNull because it throws NullPointerException; we would prefer to throw AssertionError or IllegalStateException
• Replace x.equals("") and x.size() == 0 with x.isEmpty()
• Make fields final where possible
• Make private methods and inner classes static where possible
• In class Pair<K, V> make the type variables K and V no longer nullable by default (you can make each of them nullable if you need)

[sonarqubecloud]

Quality Gate passed

Issues
35 New issues
0 Accepted issues

Measures
0 Security Hotspots
69.2% Coverage on New Code
1.4% Duplication on New Code

See analysis details on SonarCloud