Fix behaviour around Option<>s

Signed-off-by: Olivier <olivier@librepush.net>

formbeam_derive/src/derive_form.rs

@@ -318,7 +318,14 @@ fn write_partial_impl_validate(
 
 struct FieldInfo {
     ftype: FieldType,
+    /// Whether the field is required to be present in the form.
+    /// This is true unless the type is `Option<T>`.
     needed: bool,
+    /// Whether the field needs to be non-empty. By default this is true.
+    /// If `needed` is false and `need_nonempty` is true, then empty fields will be coerced to `None` instead.
+    /// If both `needed` and `need_nonempty` are false, then empty fields will be `Some("")` o.e.; only absent fields will be `None`.
+    /// If both `needed` and `need_nonempty` are true, then empty fields will not be accepted
+    /// and the `Required` validator is added to the field's info (this matches HTML form semantics).
     need_nonempty: bool,
     min_chars: Option<u32>,
     max_chars: Option<u32>,
@@ -353,7 +360,7 @@ fn write_form_info_for_field(field: &Field, f_info: &FieldInfo) -> TokenStream {
     let name = field.ident.as_ref().unwrap().to_string();
     let mut validators = Vec::new();
 
-    if f_info.need_nonempty {
+    if f_info.needed && f_info.need_nonempty {
         validators.push(quote!(::formbeam::FieldValidatorInfo::Required));
     }
 
@@ -471,6 +478,16 @@ fn write_partial_impl_form_method(
 
             if field_info.needed {
                 converter
+            } else if field_info.need_nonempty {
+                // For not-needed fields that must not be empty,
+                // coerce empty fields into `None`
+                quote!(
+                    if raw.is_empty() {
+                        None
+                    } else {
+                        Some({#converter})
+                    }
+                )
             } else {
                 quote!(Some({#converter}))
             }