Eliminate nullness errors from GenerateSequence

[kennknowles]

The inconsistent nullness annotations on the autovalue for GenerateSequence cause nondeterministic code generation. This in turn made it impossible to get to the point of reproducing a flake I was tracking down.

This PR simply eliminates the existing nullness errors and makes this particular file consistent, removing the suppression of the type checker.

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[johnjcasey]

LGTM

[reuvenlax]

Unfortunately this PR seems to have broken tests :( Somehow it's caused AutoValue to decide that maxReadTime was a required parameter, which was not the case before).

[kennknowles]

Amusingly, that is exactly why I opened this PR. Previously, local to my machine, autovalue decided that maxReadTime (and all the others) were required parameters.

[kennknowles]

Is there a revert or a forward fix I can LGTM?

[kennknowles]

Here is an existing autovalue issue that is what happened here: google/auto#1338