feat: mark IgnoreNulls and RespectNulls as unsupported on postgres and mysql

Fixes #6376

Author: NickCrews

sqlglot/dialects/mysql.py

@@ -1329,6 +1329,11 @@ def attimezone_sql(self, expression: exp.AtTimeZone) -> str:
 
         def isascii_sql(self, expression: exp.IsAscii) -> str:
             return f"REGEXP_LIKE({self.sql(expression.this)}, '^[[:ascii:]]*$')"
+        
+        def ignorenulls_sql(self, expression: exp.IgnoreNulls) -> str:
+            # https://dev.mysql.com/doc/refman/8.4/en/window-function-descriptions.html
+            self.unsupported("MySQL does not support IGNORE NULLS.")
+            return self.sql(expression.this)
 
         @unsupported_args("this")
         def currentschema_sql(self, expression: exp.CurrentSchema) -> str:

sqlglot/dialects/postgres.py

@@ -837,6 +837,16 @@ def computedcolumnconstraint_sql(self, expression: exp.ComputedColumnConstraint)
 
         def isascii_sql(self, expression: exp.IsAscii) -> str:
             return f"({self.sql(expression.this)} ~ '^[[:ascii:]]*$')"
+        
+        def ignorenulls_sql(self, expression: exp.IgnoreNulls) -> str:
+            # https://www.postgresql.org/docs/current/functions-window.html
+            self.unsupported("PostgreSQL does not support IGNORE NULLS.")
+            return self.sql(expression.this)
+        
+        def respectnulls_sql(self, expression: exp.RespectNulls) -> str:
+                        # https://www.postgresql.org/docs/current/functions-window.html
+            self.unsupported("PostgreSQL does not support explicit RESPECT NULLS. Just use the expression directly.")
+            return self.sql(expression.this)
 
         @unsupported_args("this")
         def currentschema_sql(self, expression: exp.CurrentSchema) -> str: