[ggj][ast][engx] fix: validate non-null elements for WhileStatement (#469)

* fix: swap assertEquals args in JavaWriterVisitorTest to match (expected, actusl) order

* fix: swap assertEquals args in ImportWriterVisitorTest to match (expected, actusl) order

* fix: add node validator to refactor/centralize null element checks

* fix: validate non-null elements for IfStatement

* fix: validate non-null elements for AnonymousClassExpr

* fix: validate non-null elements for BlockStatement

* fix: validate non-null elements for ClassDefinition

* fix: validate non-null elements for ConcreteReference

* fix: validate non-null elements for ForStatement

* fix: validate non-null elements for GeneralForStatement

* fix: validate non-null elements for MethodDefinition

* fix: validate non-null elements for PackageInfoDefinition

* fix: validate non-null elements for SynchronizedStatement

* fix: validate non-null elements for TryCatchStatement

* fix: validate non-null elements for VaporReference

* fix: validate non-null elements for VariableExpr

* fix: validate non-null elements for WhileStatement

Author: miraleung

src/main/java/com/google/api/generator/engine/ast/WhileStatement.java

@@ -41,10 +41,14 @@ public abstract static class Builder {
 
     public abstract Builder setBody(List<Statement> body);
 
+    // Private.
+    abstract ImmutableList<Statement> body();
+
     abstract WhileStatement autoBuild();
 
     public WhileStatement build() {
       WhileStatement whileStatement = autoBuild();
+      NodeValidator.checkNoNullElements(body(), "body", "while statement");
       Preconditions.checkState(
           whileStatement.conditionExpr().type().equals(TypeNode.BOOLEAN),
           "While condition must be a boolean-typed expression");