## Proposed changes
Issue Number: close #xxx
add testcases like:
```groovy
qt_notnull1 "select covar_samp(non_nullable(x), non_nullable(y)) from test_covar_samp"
qt_notnull2 "select covar_samp(x, non_nullable(y)) from test_covar_samp"
qt_notnull3 "select covar_samp(non_nullable(x), y) from test_covar_samp"
```
before they will all coredump in 2.1