Fix new gbstats integration if no data returned

[lukesonnet]

Our recently landed PR #1823 didn't handle the case where no data was returned for 1 or more metrics.

Before that PR, if there was missing data, we just skipped the gbstats call altogether here:

growthbook/packages/back-end/src/services/stats.ts

Lines 54 to 60 in ec88dce

	if (!rows || !rows.length) {
	return {
	unknownVariations: [],
	multipleExposures: 0,
	dimensions: [],
	};
	}

After that PR, we removed that check, and were trying to process data with 0 rows in python which was casuing key errors, like this one: https://growthbookapp.slack.com/archives/C02BHUS8NBE/p1702998839752619

Now, we are spinning up python once per analysis, so we need to handle the case where 1 or more metrics are missing data when looping over metrics, which is in gbstats now.

Loom with before and after: https://www.loom.com/share/55099e1de1df400c88de06e77eada25c

[github-actions]

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