Change types of filter_list and default_path to Option

Passing None (and hence std::ptr::nul()) is better than
allocating an empty string. Update example.rs and README.md
to reflect this.

Author: saurvs

README.md

@@ -21,15 +21,15 @@ Currently, only single file and save dialogs are supported, and the crate has be
   use nfd::*;
 
   fn main() {
-    let filter_str = "";
-    let default_path = "~";
-    let result = open_file_dialog(filter_str, default_path);
+
+    let result = open_file_dialog(None, None);
 
     match result {
         NFDResult::Okay(file_path) => println!("File path = {:?}", file_path),
         NFDResult::Cancel => println!("User canceled"),
         NFDResult::Error(error) => println!("Error: {}", error),
     }
+
   }
   ```
 

examples/example.rs

@@ -3,9 +3,7 @@ extern crate nfd;
 use nfd::*;
 
 fn main() {
-    let filter_str = "";
-    let default_path = "~";
-    let result = open_file_dialog(filter_str, default_path);
+    let result = open_file_dialog(None, None);
 
     match result {
         NFDResult::Okay(file_path) => println!("File path = {:?}", file_path),

src/lib.rs

@@ -23,32 +23,48 @@ enum DialogType {
 
 /// Open single file dialog
 #[inline(always)]
-pub fn open_file_dialog(filter_list: &str, default_path: &str) -> NFDResult {
+pub fn open_file_dialog(filter_list: Option<&str>, default_path: Option<&str>) -> NFDResult {
     open_dialog(filter_list, default_path, &DialogType::SingleFile)
 }
 
 /// Open save dialog
 #[inline(always)]
-pub fn open_save_dialog(filter_list: &str, default_path: &str) -> NFDResult {
+pub fn open_save_dialog(filter_list: Option<&str>, default_path: Option<&str>) -> NFDResult {
     open_dialog(filter_list, default_path, &DialogType::SaveFile)
 }
 
-fn open_dialog(filter_list: &str, default_path: &str, dialog_type: &DialogType) -> NFDResult {
+fn open_dialog(filter_list: Option<&str>, default_path: Option<&str>, dialog_type: &DialogType) -> NFDResult {
     let result: nfdresult_t;
     let result_cstring;
 
-    let filter_list_cstring = CString::new(filter_list).unwrap();
-    let default_path_cstring = CString::new(default_path).unwrap();
+    let filter_list_cstring;
+    let filter_list_ptr = match filter_list {
+        Some(fl_str) => {
+            filter_list_cstring = CString::new(fl_str).unwrap();
+            filter_list_cstring.as_ptr()
+        }
+        None => std::ptr::null()
+    };
+
+    let default_path_cstring;
+    let default_path_ptr = match default_path {
+        Some(dp_str) => {
+            default_path_cstring = CString::new(dp_str).unwrap();
+            default_path_cstring.as_ptr()
+        }
+        None => std::ptr::null()
+    };
+
     let out_path = CString::new("").unwrap().into_raw() as *mut *mut c_char;
 
     unsafe {
         result = match dialog_type {
             &DialogType::SingleFile => {
-                NFD_OpenDialog(filter_list_cstring.as_ptr(), default_path_cstring.as_ptr(), out_path)
+                NFD_OpenDialog(filter_list_ptr, default_path_ptr, out_path)
             },
 
             &DialogType::SaveFile => {
-                NFD_SaveDialog(filter_list_cstring.as_ptr(), default_path_cstring.as_ptr(), out_path)
+                NFD_SaveDialog(filter_list_ptr, default_path_ptr, out_path)
             },
         };