Re: BUG #19102: Assertion failure in generate_orderedappend_paths with aggregate pushdown

Server crashes with assertion failure:
TRAP: FailedAssertion("childrel->rows > 0", File: "allpaths.c", Line: 1983)

To fix the issue, we can replace the direct division with
clamp_row_est(childrel->rows) to safely handle zero, and remove the
incorrect assertion:

-   Assert(childrel->rows > 0);
-   path_fraction /= childrel->rows;
+   path_fraction /= clamp_row_est(childrel->rows);