checker, cgen: fix generics call with interface arg(fix #19976)

[shove70]

1. Fixed arrays.concat doesn't work as expected with empty interface types #19976
2. Add tests.

import arrays

interface AnythingGoes{ }

mut the_one_array := []AnythingGoes{}

some_string := 'It would suck'
another_string := 'if I was forced'
last_string := 'to << every var into the array'
random_int := 69

// this produces an error, but it shouldn't!
the_one_array = arrays.concat(the_one_array, some_string, another_string, last_string, random_int)

println(the_one_array)

outputs:

[AnythingGoes('It would suck'), AnythingGoes('if I was forced'), AnythingGoes('to << every var into the array'), AnythingGoes(69)]

[spytheman]

Good work.