internal/colltab: fix zero digit

[xlucas]

Produce a digit instead of nothing when a numeric value contains only zeros.

Fixes golang/go#67997

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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[gopherbot]

Message from Xavier Lucas:

Patch Set 2:

(1 comment)

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[gopherbot]

Message from Xavier Lucas:

Patch Set 2:

(1 comment)

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