Fixes https://github.com/googleapis/nodejs-datastore/issues/787 🦕

As described in https://github.com/googleapis/nodejs-datastore/issues/787, nodejs-datastore raises TypeError: Cannot read property 'entityValue' of undefined if we try to exclude undefined properties from index using .*.

This is because we didn't check the property existence (e.g. for non_exist_data.*, we should check non_exist_data's existence), so this PR make entity.ts check their existence before traversing data for excludeFromIndexes with a wildcard.

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@googlebot I signed it!

@crwilcox or @stephenplusplus could I trouble y'all to have a look here?

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Can someone please add this fix already? It is quite the detrimental bug.

It is a huge shame that this is still broken today in 7.0.0. In fact, two years ago my colleague posted about the same issue and the issue was closed with inaccurate information.

Closed in favour of https://github.com/googleapis/nodejs-datastore/pull/1114.