[27.x backport] builder: fall back to defaultKeepStorage if keepStorage is unset for GC policy

[thaJeztah]

• backport builder: fall back to defaultKeepStorage if keepStorage is unset for GC policy #49062

(cherry picked from commit b08ff81)

- What I did

Builder GC policies without a keepStorage option set should fall back to using the defaultKeepStorage value. But currently, the policy's keepStorage is parsed unconditionally, meaning that if it's unset, an error occurs:

could not get builder GC policy: invalid size: ''

The only way to get a policy to use defaultKeepStorage limit for GC would be to set it to "0".

This patch is to make sure policy keepStorage is conditionally parsed, if set, otherwise the defaultKeepStorage is used.

I also modified related error messages:

could not get builder GC policy: failed to parse defaultKeepStorage: invalid size: 'foo'
could not get builder GC policy: failed to parse keepStorage: invalid size: 'bar'

- How I did it

- How to verify it

Tested manually;

{
        "builder": {
                "gc": {
                        "enabled": true,
                        "defaultKeepStorage": "10GB",
                        "policy": [ { "filter": ["unused-for=2200h"] } ] 
                }
        }
}

$ docker buildx inspect
...
GC Policy rule#0:
 All:            false
 Filters:        
 Keep Duration:  2200h0m0.5074347s
 Reserved Space: 10GiB

- Description for the changelog

- Builder GC policies without a `keepStorage` value now inherit the `defaultKeepStorage` limit as intended.

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