Add check for string type for getVariableDescriptors function

[doubleUTF]

Fixes variable issue when incorporating data source with different query type.

To recreate: Include another data source such as Prometheus: https://prometheus.io/docs/prometheus/latest/getting_started/ Create a template variable based on Prometheus data source, use any valid query such as:

Proof:

[mayurOciSA]

looks good