pkl-parser: Represent "no children" as an empty list instead of null (#1513)

Motivation:
Facilitate the use of the NullAway checker as part of moving to
JSpecify.

Changes:
- represent "no children" as `List.of()` instead of null
- remove obsolete `children != null` assertions
  - NullAway intentionally ignores such assertions
- remove "no children" special-casing where no longer necessary

Result:
- cleaner code with similar performance
- removed a barrier to using the NullAway checker

pkl-core/src/main/java/org/pkl/core/runtime/PowerAssertions.java

@@ -219,9 +219,6 @@ public class PowerAssertions {
       return parserNode;
     }
     var children = parserNode.children();
-    if (children == null) {
-      return null;
-    }
     for (var child : children) {
       var found = findParserNode(node, child, offset);
       if (found != null) {