Allow expr_to_sql unparsing with no quotes (apache#10198)

* Allow expr_to_sql unparsing with no quotes

* Add test

datafusion/sql/src/unparser/dialect.rs

@@ -26,7 +26,7 @@ pub struct DefaultDialect {}
 
 impl Dialect for DefaultDialect {
     fn identifier_quote_style(&self) -> Option<char> {
-        None
+        Some('"')
     }
 }
 

datafusion/sql/src/unparser/expr.rs

@@ -358,7 +358,7 @@ impl Unparser<'_> {
     pub(super) fn new_ident(&self, str: String) -> ast::Ident {
         ast::Ident {
             value: str,
-            quote_style: Some(self.dialect.identifier_quote_style().unwrap_or('"')),
+            quote_style: self.dialect.identifier_quote_style(),
         }
     }
 
@@ -965,4 +965,20 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn custom_dialect_none() -> Result<()> {
+        let dialect = CustomDialect::new(None);
+        let unparser = Unparser::new(&dialect);
+
+        let expr = col("a").gt(lit(4));
+        let ast = unparser.expr_to_sql(&expr)?;
+
+        let actual = format!("{}", ast);
+
+        let expected = r#"(a > 4)"#;
+        assert_eq!(actual, expected);
+
+        Ok(())
+    }
 }