Fix empty parenthesized type unexpected error (#1323)

Author: spyoungtech

pkl-core/src/test/files/LanguageSnippetTests/input/errors/emptyParenthesizedTypeAnnotation.pkl

@@ -0,0 +1 @@
+foo: Int|()|String = "foo"

pkl-core/src/test/files/LanguageSnippetTests/output/errors/emptyParenthesizedTypeAnnotation.err

@@ -0,0 +1,6 @@
+–– Pkl Error ––
+Unexpected token `)`. Expected a type.
+
+x | foo: Int|()|String = "foo"
+              ^
+at emptyParenthesizedTypeAnnotation (file:///$snippetsDir/input/errors/emptyParenthesizedTypeAnnotation.pkl)

pkl-parser/src/main/java/org/pkl/parser/Parser.java

@@ -1393,6 +1393,9 @@ private Type parseTypeAtom(@Nullable String expectation) {
           children.add(ret);
           typ = new Type.FunctionType(children, tk.span.endWith(ret.span()));
         } else {
+          if (children.isEmpty()) {
+            throw new ParserError(ErrorMessages.create("unexpectedTokenForType", ")"), end);
+          }
           typ = new ParenthesizedType((Type) children.get(0), tk.span.endWith(end));
         }
       }

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

@@ -105,6 +105,7 @@ class ParserComparisonTest {
         "errors/moduleWithHighMinPklVersionAndParseErrors.pkl",
         "errors/underscore.pkl",
         "errors/shebang.pkl",
+        "errors/emptyParenthesizedTypeAnnotation.pkl",
         "notAUnionDefault.pkl",
         "multipleDefaults.pkl",
         "modules/invalidModule1.pkl",