Use Null to replace the results of some Math functions that return NaN.

Among Calcite's Math functions, there are some functions that return double types. They are calculated using Java's Math method in SqlFunctions, so when the result is empty, NaN will be returned, but according to SQL rules, NULL should be returned.

e.g

f.checkScalarApprox("sqrt(-1)", "DOUBLE NOT NULL",
    "NaN");