Annotate the rest of the main package (basically just the Java 8 subjects) for nullness.

And move the `@NullMarked` annotation from individual classes up to the package.

Motivation:
- The annotating is about potentially making things nicer for callers (or any future J2KT use?).
- Moving `@NullMarked` to the package is about saving Truth users from [a warning when running Error Prone with `--release 8`](#1320): When `@NullMarked` appears on a _class_, Error Prone processes the entire `NullMarked` class in order to check whether `NullMarked` is `@Inherited`. This leads to `warning: unknown enum constant ElementType.MODULE` because `NullMarked` has `@Target(MODULE, ...)` but `MODULE` isn't available until Java 9. We'll also be fixing this on the Error Prone side, but we might as well work around it on the Truth side, too—and annotate the rest of Truth while we're at it.

(In principle, I should now add `@NullUnmarked` to all of Truth's test classes, since they haven't been annotated. But doing so would have little to no effect in practice unless maybe IntelliJ recognizes `@NullUnmarked` (probably not now?) or we improve our nullness offerings in Error Prone. I'm planning to not bother, but let me know if I'm being overly lazy.)

Note that this CL applies `@Nullable` to some assertion methods' parameters even though those assertions would always fail if the callers passed `null`. This follows a principle that we'd applied (albeit incompletely) in cl/516515683, which showed that such changes avoided producing build errors in existing, working code. The principle is the same as that discussed for `EqualsTester` in cl/578260904.

Fixes #1320

RELNOTES=Annotated the rest of the main package for nullness, and moved the `@NullMarked` annotation from individual classes up to the package to avoid [a warning under `--release 8`](#1320).
PiperOrigin-RevId: 651891918

Author: cpovirk

core/src/main/java/com/google/common/truth/AbstractArraySubject.java

@@ -20,15 +20,13 @@
 import static com.google.common.truth.Fact.simpleFact;
 
 import java.lang.reflect.Array;
-import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
 
 /**
  * A common supertype for Array subjects, abstracting some common display and error infrastructure.
  *
  * @author Christian Gruber (cgruber@israfil.net)
  */
-@NullMarked
 abstract class AbstractArraySubject extends Subject {
   private final @Nullable Object actual;
 

core/src/main/java/com/google/common/truth/ActualValueInference.java

@@ -33,7 +33,6 @@
 import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.Map.Entry;
-import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
 import org.objectweb.asm.ClassReader;
 import org.objectweb.asm.ClassVisitor;
@@ -70,7 +69,6 @@
  */
 @GwtIncompatible
 @J2ktIncompatible
-@NullMarked
 final class ActualValueInference {
   /** <b>Call {@link Platform#inferDescription} rather than calling this directly.</b> */
   static @Nullable String describeActualValue(String className, String methodName, int lineNumber) {