fix(spans): add Span::empty() arm for CreateAggregate

PR #7 added the Statement::CreateAggregate variant but omitted the
corresponding match arm in the Spanned impl for Statement. Fork CI
never ran on the PR so this was not caught before merge.

Author: fmguerreiro

src/ast/spans.rs

@@ -521,6 +521,7 @@ impl Spanned for Statement {
             Statement::Vacuum(..) => Span::empty(),
             Statement::AlterUser(..) => Span::empty(),
             Statement::Reset(..) => Span::empty(),
+            Statement::CreateAggregate(_) => Span::empty(),
         }
     }
 }