Add RedundantNullCheck bug pattern.

[copybara-service]

Add RedundantNullCheck bug pattern.

Closes #5107.

A null check is redundant if it is performed on a variable or method call that is known to be non-null within a @NullMarked scope.

Within a @NullMarked scope, types are non-null by default unless explicitly annotated with @Nullable.
Therefore, checking a variable or method return value (that isn't @Nullable) for nullness is unnecessary, as it should never be null.

Example:

import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

@NullMarked
class MyClass {
  void process(String definitelyNonNull) {
    // BUG: Diagnostic contains: RedundantNullCheck
    if (definitelyNonNull == null) {
      System.out.println("This will never happen");
    }
    // ...
  }

  String getString() {
    return "hello";
  }

  @Nullable String getNullableString() {
    return null;
  }

  void anotherMethod() {
    String s = getString();
    // BUG: Diagnostic contains: RedundantNullCheck
    if (s == null) {
      // s is known to be non-null because getString() is not @Nullable
      // and we are in a @NullMarked scope.
      System.out.println("Redundant check");
    }

    String nullableStr = getNullableString();
    if (nullableStr == null) { // This check is NOT redundant
      System.out.println("Nullable string might be null");
    }
  }
}

This check helps to clean up code and reduce clutter by removing unnecessary null checks, making the code easier to read and maintain.
It also reinforces the contract provided by @NullMarked and @Nullable annotations.

Fixes #5121

FUTURE_COPYBARA_INTEGRATE_REVIEW=#5121 from bdragan:5107-redundant_null_check f935970