Fix crash due to parse error in module header (#1112)

pkl-core/src/test/files/LanguageSnippetTests/input/modules/invalidModule1.pkl

@@ -0,0 +1 @@
+module "DoNotQuote"

pkl-core/src/test/files/LanguageSnippetTests/output/modules/invalidModule1.err

@@ -0,0 +1,6 @@
+–– Pkl Error ––
+Unexpected token `"`. Expected `identifier`.
+
+x | module "DoNotQuote"
+           ^
+at invalidModule1 (file:///$snippetsDir/input/modules/invalidModule1.pkl)

pkl-parser/src/main/java/org/pkl/parser/syntax/Module.java

@@ -33,7 +33,8 @@ public final class Module extends AbstractNode {
 
   public @Nullable ModuleDecl getDecl() {
     assert children != null;
-    return (ModuleDecl) children.get(0);
+    // children can be empty when there was a parse error
+    return children.isEmpty() ? null : (ModuleDecl) children.get(0);
   }
 
   public List<ImportClause> getImports() {

pkl-parser/src/test/kotlin/org/pkl/parser/ParserComparisonTest.kt

@@ -91,6 +91,7 @@ class ParserComparisonTest : ParserComparisonTestInterface {
         "errors/underscore.pkl",
         "notAUnionDefault.pkl",
         "multipleDefaults.pkl",
+        "modules/invalidModule1.pkl",
       )
 
     private val regexExceptions =