Fix formatting of nodes with no children (#1351)

For example, this fixes an issue where an empty module turns into ` \n`.

Closes https://github.com/apple/pkl/issues/1348

---------

Co-authored-by: Jen Basch <jbasch94@gmail.com>

pkl-formatter/src/test/kotlin/org/pkl/formatter/FormatterTest.kt

@@ -113,4 +113,11 @@ class FormatterTest {
 
     walkDir(outputDir)
   }
+
+  @Test
+  fun `whitespace only`() {
+    for (src in listOf(";;;", "\n", "\n\n\n", "\t")) {
+      assertThat(format(src)).isEqualTo("\n")
+    }
+  }
 }