Kibana should allow a min_age setting of 0ms in ILM policy phases

[newtonne]

Kibana version:
7.4.1
Elasticsearch version:
7.4.1
Server OS version:
Windows 10
Browser version:
Chrome Version 78.0.3904.87
Original install method (e.g. download page, yum, from source, etc.):
yum
Describe the bug:
Kibana doesn't allow the creation (or amendment) of an ILM policy without specifying a min_age greater than 0 for each phase. It should be possible to create an ILM policy with phases that have min_age=0 by either not setting a value, or explicitly setting it to 0. Neither of these is currently possible.

Steps to reproduce:

1. Go to Management => Index Lifecycle Policies => Create Policy
2. Type a policy name
3. Activate the cold phase
4. Save as new policy => ERROR (a number is required)
5. Set Timing for cold phase to 0 => ERROR (Only numbers above 0 are allowed.)

Expected behavior:

If the value is unset, Kibana should send the JSON request with the value omitted.
If the value is set to 0, Kibana should send the JSON request with the value set to 0.

Any additional context:

In the ILM docs it is mentioned that min_age is optional and defaults to 0ms:

the hot phase defined in the "phases" section. Optional min_age field not defined — defaults to 0ms

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)