Add is_null getter for ColumnDef struct (#1381)

* Add is_null getter for ColumnDef struct

* Update src/entity/column.rs

* Update src/entity/column.rs

Co-authored-by: Chris Tsang <chris.2y3@outlook.com>

src/entity/column.rs

@@ -401,6 +401,11 @@ impl ColumnDef {
     pub fn get_column_type(&self) -> &ColumnType {
         &self.col_type
     }
+
+    /// Returns true if the column is nullable
+    pub fn is_null(&self) -> bool {
+        self.null
+    }
 }
 
 impl From<ColumnType> for sea_query::ColumnType {