Parser should not throw an error if predicate not present

[sc07kvm]

Context

A change introduced in #842 (related to #841) altered the previous behavior. The earlier behavior was intentional: functions can be provided via env at runtime.

If the following code works:

program, err := expr.Compile(`fn(1)`)
out, err := expr.Run(program, map[string]any{"fn": func(a int) int { return a }})

then this code should work as well:

program, err := expr.Compile(`upper(1)`, expr.DisableBuiltin("upper"))
out, err := expr.Run(program, map[string]any{"upper": func(a int) int { return a }})

Compatibility

#842 also introduced a backward-incompatible behavior in a minor release.