Fix error on formatting annotation for primary constructor

ktlint-ruleset-experimental/src/main/kotlin/com/pinterest/ktlint/ruleset/experimental/AnnotationRule.kt

@@ -94,7 +94,8 @@ class AnnotationRule : Rule("annotation") {
             // text in the file
             val newLineWithIndent = (nodeBeforeAnnotations?.text ?: "\n").let {
                 // Make sure we only insert a single newline
-                it.substring(it.lastIndexOf('\n'))
+                if (it.contains('\n')) it.substring(it.lastIndexOf('\n'))
+                else it
             }
 
             if (noWhiteSpaceAfterAnnotation) {

ktlint-ruleset-experimental/src/test/kotlin/com/pinterest/ktlint/ruleset/experimental/AnnotationRuleTest.kt

@@ -440,4 +440,19 @@ class AnnotationRuleTest {
             """.trimIndent()
         )
     }
+
+    @Test
+    fun `no error with formatting annotation for primary constructor`() {
+        val code =
+            """
+            class Foo @Inject internal constructor()
+            """.trimIndent()
+        assertThat(
+            AnnotationRule().format(code)
+        ).isEqualTo(
+            """
+            class Foo @Inject internal constructor()
+            """.trimIndent()
+        )
+    }
 }