fix(discover): Fix `has:measurements.*` queries

[gggritso]

Fixes SENTRY-3DB9

We're seeing Snuba errors for comparisons of a numeric value to a string. e.g.,

`measurements.frames_frozen_rate` != ''

Measurements are numeric, so this errors out. This comparison is created for queries like has:measurements.frozen_frame_rate. That measurement is "synthetic", it's a function that divides two other measurements. It returns a numeric value, so comparison to string fails. The query builder's current logic doesn't account for measurement functions, and creates the comparison incorrectly.

The attempted fix is to check for measurements, and return early with a basic check for null.