compute: Compute returns empty Value instead of replacement

Created by: coury-clark

Sometimes it seems we get an empty Value field in a compute replacement, even when the replacement is a static string. Example:

fork:yes archived:yes lang:go count:99999999 content:output(historical -> yayaya) repo:^(github\.com/sourcegraph/sourcegraph)$

As I understand, this should result in every match being replaced with yayaya. Instead, some of the results are all empty Value.

event: results
data: [{"value":"yayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"}]

event: progress
data: {"done":false,"matchCount":0,"durationMs":39,"skipped":[]}

event: results
data: [{"value":"yayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\nyayaya\nyayaya\nyayaya\nyayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\nyayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\nyayaya\nyayaya\nyayaya\nyayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\nyayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\nyayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\nyayaya\nyayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"yayaya\n","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"}]

event: progress
data: {"done":false,"matchCount":0,"durationMs":103,"skipped":[]}

// This is the notable section
event: results
data: [{"value":"","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"},{"value":"","kind":"output","repositoryID":11,"repository":"github.com/sourcegraph/sourcegraph"}]

event: progress
data: {"done":false,"matchCount":0,"durationMs":125,"skipped":[]}

event: progress
data: {"done":true,"matchCount":0,"durationMs":125,"skipped":[]}

event: done
data: {}

/cc @benvenker @lguychard