In COPY FROM, fail cleanly when unsupported encoding conversion is needed.

In recent releases, such cases fail with "cache lookup failed for
function 0" rather than complaining that the conversion function
doesn't exist as prior versions did.  Seems to be a consequence of
sloppy refactoring in commit f82de5c46.  Add the missing error check.

Per report from Pierre Fortin.  Back-patch to v14 where the
oversight crept in.

Discussion: https://postgr.es/m/20230929163739.3bea46e5.pfortin@pfortin.com

src/backend/commands/copyfrom.c

@@ -1481,6 +1481,12 @@ BeginCopyFrom(ParseState *pstate,
 		cstate->need_transcoding = true;
 		cstate->conversion_proc = FindDefaultConversionProc(cstate->file_encoding,
 															GetDatabaseEncoding());
+		if (!OidIsValid(cstate->conversion_proc))
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_FUNCTION),
+					 errmsg("default conversion function for encoding \"%s\" to \"%s\" does not exist",
+							pg_encoding_to_char(cstate->file_encoding),
+							pg_encoding_to_char(GetDatabaseEncoding()))));
 	}
 
 	cstate->copy_src = COPY_FILE;	/* default */