regr_r2 and potentially other statistical functions can't handle tiny negative float values as variance

The current statistical functions code doesn't take into account a minuscule negative deviation value that is actually represents a float 0.
According with the basic properties of Variance [1] and the regr_r2 calculation algo [2] the covar_popy must be non-negative so that sqrt(covar_popy) in regr_r2::evaluate() doesn't return NaN.

4QA This query must return 0.0000000000 according with the reference

{ select round(regr_r2(l_tax, l_extendedprice),10) from lineitem where l_tax = 0.05 group by l_tax order by l_tax; }

1. https://en.wikipedia.org/wiki/Variance
2. https://docs.oracle.com/cd/B28359_01/server.111/b28286/functions139.htm#SQLRF00696