`AddNullMarkedToPackageInfo` should also report absent packages

# Problem

There is an experimental `AddNullMarkedToPackageInfo` bug pattern which reports `package-info.java` files which do not have `@NullMarked` applied to them.

It does help detect the issue of forgetting to add `@NullMarked` to the package, but in our experience a more common problem is forgetting to create package-info itself and thus marking it with `@NullMarked` which is currently *not* caught by the checker.

# Solution

The suggestion is to also handle this scenario, probably also allowing the package to be explicitly `@NullUnmarked` so that there still is an escape hatch for unannotated packages.

## Pseudo-code

Currently:

```java
if (thePackage.exists()) {
if (!thePackage.isAnnotated(NullMarked.class)) {
reportError("No @NullMarked on packages %s", thePackage);
}
}
```

Suggested:

```java
if (thePackage.exists()) {
if (!thePackage.isAnnotated(NullMarked.class)
&& !thePackage.isAnnotated(NullUnmarked.class)) {
reportError("No @NullMarked or @NullUnmarked on packages %s", thePackage);
}
} else {
reportError("No package info %s", thePackage);
}
```