Simplify return type of Data::print_string

Change the return type from `Option<String>` to `String` because
libyang guarantees that a valid string is always returned on success.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

src/data.rs

@@ -306,7 +306,7 @@ pub trait Data<'a> {
         &self,
         format: DataFormat,
         options: DataPrinterFlags,
-    ) -> Result<Option<String>> {
+    ) -> Result<String> {
         let mut cstr = std::ptr::null_mut();
         let cstr_ptr = &mut cstr;
 
@@ -322,15 +322,15 @@ pub trait Data<'a> {
             return Err(Error::new(self.context()));
         }
 
-        Ok(char_ptr_to_opt_string(cstr))
+        Ok(char_ptr_to_string(cstr))
     }
 
     /// Print data tree in the specified format to a bytes vector.
     fn print_bytes(
         &self,
         format: DataFormat,
         options: DataPrinterFlags,
-    ) -> Result<Option<Vec<u8>>> {
+    ) -> Result<Vec<u8>> {
         let mut cstr = std::ptr::null_mut();
         let cstr_ptr = &mut cstr;
 
@@ -346,10 +346,6 @@ pub trait Data<'a> {
             return Err(Error::new(self.context()));
         }
 
-        if cstr.is_null() {
-            return Ok(None);
-        }
-
         let bytes = match format {
             DataFormat::XML | DataFormat::JSON => {
                 // Convert the null-terminated C string to a vector of bytes.
@@ -369,7 +365,7 @@ pub trait Data<'a> {
                     .to_vec()
             }
         };
-        Ok(Some(bytes))
+        Ok(bytes)
     }
 }