NOTEQUAL filter selectivity is incorrect when source NDV is 1

[aaneja]

Setup

1. Source variable with null_fraction as 0, and NDV count as 1
2. Input plan node has 100 rows
3. A filter like name <> 'foo'

Expected behavior

Filter selectivity is non-zero

Observed behavior

Filter selectivity is 0, the planner assumes no rows will be selected
See aaneja@31732a8 for a repro test case