sql/stats: store non-NULL histograms for empty tables

[michae2]

We have been storing NULL histograms / using nil HistogramData for all
the following cases:

1. regular stats, GenerateHistogram=false
2. regular stats, GenerateHistogram=true, empty table
3. regular stats, GenerateHistogram=true, all NULL values
4. inverted stats, no inverted index
5. inverted stats, yes inverted index, empty table
6. inverted stats, yes inverted index, all NULL values

When predicting histograms for statistics forecasts, we need to
distinguish between case 1 and cases 2 and 3. In case 1 we cannot
predict histograms, but in cases 2 and 3 we can (and the emptiness of
the histogram is important).

So, for cases 2 and 3 we now store an empty histogram instead of NULL,
and correspondingly use an initialized HistogramData with 0-length
Buckets instead of nil HistogramData.

This also helps with testing statistics forecasts.

(The inability to distinguish cases 4-6 doesn't matter, because we
cannot predict histograms for inverted indexes anyway. I tried to change
cases 5 and 6 to be non-NULL for consistency but ran into some
problems, so I'll leave them as they are.)

Release note: None

[cockroach-teamcity]

This change is 

[msirek]

Nice change!

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @rytaft)

[rytaft]

cool!

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @michae2)

[michae2]

TFTRs!

bors r=msirek,rytaft

[michae2]

Sigh.

bors r-

[craig]

Canceled.

[michae2]

Let's try this again.

bors r=msirek,rytaft

[craig]

Build succeeded:

• GitHub CI (Cockroach)

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