Provide a FORCE NULL option to COPY in CSV mode.

This forces an input field containing the quoted null string to be
returned as a NULL. Without this option, only unquoted null strings
behave this way. This helps where some CSV producers insist on quoting
every field, whether or not it is needed. The option takes a list of
fields, and only applies to those columns. There is an equivalent
column-level option added to file_fdw.

Ian Barwick, with some tweaking by Andrew Dunstan, reviewed by Payal
Singh.

src/backend/parser/gram.y

@@ -2548,6 +2548,10 @@ copy_opt_item:
 				{
 					$$ = makeDefElem("force_not_null", (Node *)$4);
 				}
+			| FORCE NULL_P columnList
+				{
+					$$ = makeDefElem("force_null", (Node *)$3);
+				}
 			| ENCODING Sconst
 				{
 					$$ = makeDefElem("encoding", (Node *)makeString($2));