[APM] Agent span_frames_min_duration configuration input cannot handle 0 or -1

[estolfo]

Kibana version:
7.7.0-BC2

Elasticsearch version:
7.7.0-BC2

Describe the bug:
The UI can't handle the values 0 and -1 right now for the span_frames_min_duration agent configuration field. Both are valid values for the Ruby and Go agents.
The interface shows an error: Please specify an integer and a unit if either value is entered.
For 0: The UI allows entering in the value 0 but the error message is displayed.
For -1: The UI doesn't allow typing a number starting with -

Steps to reproduce:

1. Create agent config entry
2. Navigate to "Span frames minimum duration"
3. Type 0 or -1, or use the arrows to attempt to enter in either of those values

Expected behavior:
The interface should allow 0 and -1 as valid values.

Screenshots (if relevant):
For 0:

For -1:

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)