[C#] No errors throwed when indexing non-existent `null` keys in a map

[Withered-Flower-0422]

MM:

]a = {}
]a[null]
]

CL:

> a = {}
> a[null]
Runtime Error:  [line 1]

a[null] won't throw an error even if there's no null key in map a in MM.
It does throw an error in CL, but the description is so vague.