fix issue https://github.com/OpenAPITools/openapi-generator/issues/12738 (#13495)

Co-authored-by: Christopher Thonfeld-Guckes <ctg@triarc-labs.com>

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/mustache/TitlecaseLambda.java

@@ -61,7 +61,15 @@ public class TitlecaseLambda implements Mustache.Lambda  {
     }
 
     private String titleCase(final String input) {
-        return input.substring(0, 1).toUpperCase(Locale.ROOT) + input.substring(1);
+        if(input == null || "".equals(input)){
+            return "";
+        }
+
+        String firstLetter = input.substring(0, 1).toUpperCase(Locale.ROOT);
+        if (input.length() == 1) {
+            return firstLetter;
+        }
+        return firstLetter + input.substring(1);
     }
 
     @Override

modules/openapi-generator/src/test/java/org/openapitools/codegen/templating/mustache/TitlecaseLambdaTest.java

@@ -15,6 +15,24 @@ public class TitlecaseLambdaTest extends LambdaTest {
         test("Once Upon A Time", "{{#titlecase}}once upon a time{{/titlecase}}", ctx);
     }
 
+    @Test
+    public void titlecaseSingleLetterTest() {
+        // Given
+        Map<String, Object> ctx = context("titlecase", new TitlecaseLambda());
+
+        // When & Then
+        test("O", "{{#titlecase}}o{{/titlecase}}", ctx);
+    }
+
+    @Test
+    public void titlecaseEmptyStringTest() {
+        // Given
+        Map<String, Object> ctx = context("titlecase", new TitlecaseLambda());
+
+        // When & Then
+        test("", "{{#titlecase}}{{/titlecase}}", ctx);
+    }
+
     @Test
     public void titlecaseWithDelimiterTest() {
         // Given