Add support for nullable string columns, and make NSString properties nullable by default

object_schema.cpp

@@ -37,6 +37,7 @@ ObjectSchema::ObjectSchema(Group *group, const std::string &name) : name(name) {
         property.type = (PropertyType)table->get_column_type(col);
         property.is_indexed = table->has_search_index(col);
         property.is_primary = false;
+        property.is_nullable = table->is_nullable(col) || property.type == PropertyTypeObject;
         property.table_column = col;
         if (property.type == PropertyTypeObject || property.type == PropertyTypeArray) {
             // set link type for objects and arrays

object_store.cpp

@@ -197,7 +197,7 @@ void ObjectStore::update_column_mapping(Group *group, ObjectSchema &target_schem
 }
 
 static inline bool property_has_changed(Property &p1, Property &p2) {
-    return p1.type != p2.type || p1.name != p2.name || p1.object_type != p2.object_type;
+    return p1.type != p2.type || p1.name != p2.name || p1.object_type != p2.object_type || p1.is_nullable != p2.is_nullable;
 }
 
 // set references to tables on targetSchema and create/update any missing or out-of-date tables
@@ -240,7 +240,7 @@ bool ObjectStore::create_tables(Group *group, ObjectStore::Schema &target_schema
                         break;
                     }
                     default:
-                        target_prop.table_column = table->add_column(DataType(target_prop.type), target_prop.name);
+                        target_prop.table_column = table->add_column(DataType(target_prop.type), target_prop.name, target_prop.is_nullable);
                         break;
                 }
                 changed = true;

property.hpp

@@ -52,6 +52,7 @@ namespace realm {
         std::string object_type;
         bool is_primary;
         bool is_indexed;
+        bool is_nullable;
 
         size_t table_column;
         bool requires_index() { return is_primary || is_indexed; }