fix(datastore): allow non-empty values in the slice

[bhshkh]

Fixes: googleapis#6983
Issue: googleapis#6983

Cause: While saving a slice of structs, we try to extract all the fields of struct from first element in array. Then, all the fields in next elements are compared to these fields. For e.g., if array is [{a: 1, b: 2, c: 3}, {a: 4, b: 5, c: 6}, {a: 1, b: 2, c: 3, d: 10}], the library will throw error that 'd' is unknown field in element 3 since it was not part of the first element.
In issue googleapis#6983, when first array element is empty, e.g. [{}, {a: 4, b: 5, c: 6}, {a: 1, b: 2, c: 3, d: 10}] or T{Values: []string{"", "s1", "s2"}}, all the later fields in later elements error out as they were not seen in first element.

Fix: Use the first non-empty element of the array as a standard for the fields in rest of the elements