[Advanced Settings] Reset to default button doesn't appear with empty strings

[markov00]

Kibana version:
I've checked that this issue is present since 6.7

Describe the bug:
In the advanced settings, when you clear (completely remove the format) a field like the Date format, the Reset to default button doesn't appear below the field. This can trick the user to think that this is the default value, causing few issues with visualizations that rely on that format.

Steps to reproduce:

1. Open the Advanced Settings
2. Go to Date Format, clean the field and save
3. theReset to default button is not present
4. now add a single character, save, the Reset to default button is there

Expected behavior:

The reset to default button should appear every time the field differs from the actual default value.

Screenshots (if relevant):

Errors in browser console (if relevant):
N/A

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)