fix: documentation trait handling empty list items (#1175)

Author: 0marperez

codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/lang/DocumentationPreprocessor.kt

@@ -104,7 +104,7 @@ class DocumentationPreprocessor : KotlinIntegration {
                 "li" -> {
                     // If this list item holds a sublist, then we essentially just want to line break right away and
                     // render the nested list as normal.
-                    val prefix = if (node.childNode(0).nodeName() == "ul") "\n" else ""
+                    val prefix = if (node.childNodes().firstOrNull()?.nodeName() == "ul") "\n" else ""
                     builder.append("$listPrefix+ $prefix")
                 }
                 "ul", "ol" -> {

codegen/smithy-kotlin-codegen/src/test/kotlin/software/amazon/smithy/kotlin/codegen/lang/DocumentationPreprocessorTest.kt

@@ -94,6 +94,17 @@ class DocumentationPreprocessorTest {
         inputTest(input, expected)
     }
 
+    @Test
+    fun `it renders lists with empty list items`() {
+        val input = "<p>an unordered list with empty list items:</p><ul><li></li><li></li></ul>"
+        val expected = """
+        an unordered list with empty list items:
+        + 
+        +
+        """.trimIndent()
+        inputTest(input, expected)
+    }
+
     @Test
     fun `it renders basic formatters`() {
         val input = "<strong>bold text</strong><br/><em>italic text</em>"