Make initial text argument optional

This allows the user to clear the initial_text field by supplying a
`None`

ctru-rs/src/applets/swkbd.rs

@@ -499,18 +499,23 @@ impl SoftwareKeyboard {
     /// use ctru::applets::swkbd::SoftwareKeyboard;
     /// let mut keyboard = SoftwareKeyboard::default();
     ///
-    /// keyboard.set_initial_text("Write here what you like!");
+    /// keyboard.set_initial_text(Some("Write here what you like!"));
     /// #
     /// # }
     #[doc(alias = "swkbdSetInitialText")]
-    pub fn set_initial_text(&mut self, text: &str) {
-        unsafe {
-            self.initial_text = Some(CString::new(text).unwrap());
+    pub fn set_initial_text(&mut self, text: Option<&str>) {
+        if let Some(text) = text {
+            let initial_text = CString::new(text).unwrap();
 
-            ctru_sys::swkbdSetInitialText(
-                self.state.as_mut(),
-                self.initial_text.as_ref().unwrap().as_ptr(),
-            );
+            unsafe {
+                ctru_sys::swkbdSetInitialText(self.state.as_mut(), initial_text.as_ptr());
+            }
+
+            self.initial_text = Some(initial_text);
+        } else {
+            unsafe { ctru_sys::swkbdSetInitialText(self.state.as_mut(), std::ptr::null()) };
+
+            self.initial_text = None;
         }
     }