Refactor user validation

Validate each user field for creation/modification via API and web UI

model/model.go

@@ -0,0 +1,21 @@
+// Copyright 2021 Frédéric Guillot. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package model // import "miniflux.app/model"
+
+// OptionalString populates an optional string field.
+func OptionalString(value string) *string {
+	if value != "" {
+		return &value
+	}
+	return nil
+}
+
+// OptionalInt populates an optional int field.
+func OptionalInt(value int) *int {
+	if value > 0 {
+		return &value
+	}
+	return nil
+}