gate Null <: TermRef under safeNulls for consistency with TypeRef

The spec for conformance currently says:
```
- `´S =´ scala.Null` and:
  - ´T = q.C[T_1, ..., T_n]´ with ´n \geq 0´ and ´C´ does not derive from `scala.AnyVal` and ´C´ is not the hidden class of an `object`, or
  - ´T = q.x´ is a term designator with underlying type ´U´ and `scala.Null ´<: U´`, or
```
https://github.com/scala/scala3/blob/10770098fb9cbdef238fc677979a19a5bfb5ba7c/docs/_spec/03-types.md?plain=1#L1497-L1499

Under explicit nulls, the former is changed only in a safe-nulls context,
but the latter is changed even in unsafe nulls. This change gates both under
safe-nulls for consistency.

Author: olhotak

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1037,10 +1037,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                 // scala.AnyRef [Scala 3: which scala.Null conforms to], the type denotes the set of values consisting
                 // of null and the value denoted by p (i.e., the value v for which v eq p). [Otherwise,] the type
                 // denotes the set consisting of only the value denoted by p.
-                !ctx.explicitNulls && isNullable(tp.underlying) && tp.isStable
+                !ctx.mode.is(Mode.SafeNulls) && isNullable(tp.underlying) && tp.isStable
               case tp: ThisType =>
                 // Same as above; this.type is also a singleton type in spec language
-                !ctx.explicitNulls && isNullable(tp.underlying)
+                !ctx.mode.is(Mode.SafeNulls) && isNullable(tp.underlying)
               case tp: RefinedOrRecType => isNullable(tp.parent)
               case tp: AppliedType => isNullable(tp.tycon)
               case AndType(tp1, tp2) => isNullable(tp1) && isNullable(tp2)