Allow tolerant mode for null string fields in TES API requests

[NeutryFD]

When using Funnel as the TES backend for Cromwell, workflow execution fails because Funnel rejecting tasks that contain null values for string fields.

Cromwell sometimes includes null for optional string metadata fields in TES requests. For example:

"tags": {
  "workflow_id": "f34e3362-2b7a-42d4-b22a-cd9ac89fe2a2",
  "root_workflow_id": "f34e3362-2b7a-42d4-b22a-cd9ac89fe2a2",
  "parent_workflow_id": null
}

This results in Funnel returning an error such as:

proto: (line X:XXXX): invalid value for string field value: null