Fix document about understanding-null-safety

[RyochanUedasan]

The example code seems to be about "without null safety".

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[RyochanUedasan]

I have just signed the Google CLA!

[parlough]

Thanks for opening a pull request and signing the CLA!

However, this snippet does appear to be "using null safety" due to the Thing? thing parameter which is marked as nullable which is only possible with null safety enabled.

What about it makes you think it's meant to be an example without null safety?

[RyochanUedasan]

@parlough Thanks for your reply.
I initially thought this code was another example of using the null-aware operator before implementing sound null safety, suggesting that ? should be used every time in a method chain, regardless of whether doohickey is non-nullable.
However, as you pointed out, Thing? thing is used here, so this is about using null safety.

I apologize for the mistake!