UI: Display minus icon for empty MaskedInput value. Show MaskedInput for KV secrets without values

[malinac02]

Description:

1. This PR changes what the <MaskedInput> component shows when it has no value

• Before: it showed just a blank space when the "show value" button is pressed.
• After: it shows a minus icon (<Icon @name="minus" />) so that it is more clear to users that there is simply no value there
  • When downloaded, the content is an empty string
  • When copied, a blank space is copied and a copy success alert shows

2. This PR changes what the KV secrets page shows when there secrets with no value

• Reproduction Steps: enable KV engine > create a path > create a secret in that path with only a key, no value > view the path so you can see the key/value pairs
• Before: secrets with only a key and no value showed the minus icon for the value.
• After: a key with no value shows the newly updated <MaskedInput>. This makes it so that the KV secrets page looks more consistent even if there are keys with no values

Pictures:
Before:

After:

[github-actions]

Build Results:
All builds succeeded! ✅

[github-actions]

CI Results:
All Go tests succeeded! ✅

[hellobontempo]

Nice work! Just a comment about test naming, but nothing blocking. 😄