[improvement] Enable NullAway gradle checks

[schlosna]

Before this PR

The safe-logging library did not explicitly annotate Arg and its subtypes, so consumers of this library would receive false-positive warnings when using static analysis checks such as NullAway or IntelliJ nullability checks, for example the below as seen in palantir/tritium#152 where use of UnsafeArg.of with a @Nullable value is mistakenly flagged:

warning: [NullAway] passing @Nullable parameter 'context' where @NonNull is required
                    UnsafeArg.of("context", context),
                                            ^

After this PR

Enables NullAway gradle build checks and appropriately annotates Arg and its subtypes to support proper nullability analysis.

Relevant to palantir/tritium#152 and supersedes #97

The NullAway checks identified one possible issue in safe-logging's LoggableExceptionAssert, and this has been corrected.

• Handle null LoggableExceptionAssert arg, addresses NullAway warnings:

> Task :preconditions-assertj:compileJava
/Volumes/git/safe-logging/preconditions-assertj/src/main/java/com/palantir/logsafe/testing/LoggableExceptionAssert.java:30: warning: [NullAway] initializer method does not guarantee @nonnull field argsAssert is initialized along all control-flow paths (remember to check for exceptions or early returns).
    public LoggableExceptionAssert(T actual) {
           ^
    (see http://t.uber.com/nullaway )
/Volumes/git/safe-logging/preconditions-assertj/src/main/java/com/palantir/logsafe/testing/LoggableExceptionAssert.java:33: warning: [NullAway] assigning @nullable expression to @nonnull field
        argsAssert = actual == null ? null : new ArgsAssert(actual.getArgs());
                   ^
    (see http://t.uber.com/nullaway )
2 warnings

Another error-prone warning identified a potential issue with Arg.equals implementation

• Arg.equals checks isSafeForLogging, addresses EqualsGetClass error-prone finding:

> Task :safe-logging:compileJava
/Volumes/git/safe-logging/safe-logging/src/main/java/com/palantir/logsafe/Arg.java:51: warning: [EqualsGetClass] Overriding Object#equals in a non-final class by using getClass rather than instanceof breaks substitutability of subclasses.
    public final boolean equals(Object other) {
                         ^
    (see https://errorprone.info/bugpattern/EqualsGetClass)
  Did you mean 'if (!(other instanceof Arg)) {'?
1 warning

[iamdanfox]

👍 I tried using IntelliJ's regenerate equals method and it gave us exactly this.

[iamdanfox]

this shouldn't change any behaviour because we don't expect people to make their own subclasses of Arg, but better safe than sorry!