ctx: Ignore empty path components in non-interactive mode

[adamwg]

Description of your changes

Allow for trailing slashes (and also, as a side-effect, duplicate slashes) in
non-interactive mode by ignoring any empty path components when we parse the
path. This is especially helpful since we print a trailing slash on the path
when a user selects a context other than a control plane (space or group).

Fixes #581

I have:

• Read and followed Upbound's contribution process.
• Run make reviewable to ensure this PR is ready for review.
  - [ ] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

Manual testing with various non-interactive invocations:

[awg/ignore-empty-trailing//up]% up ctx upbound/upbound-aws-us-east-1
Kubeconfig context "upbound": Upbound upbound/upbound-aws-us-east-1/
[awg/ignore-empty-trailing//up]% up ctx upbound/upbound-aws-us-east-1/
Kubeconfig context "upbound": Upbound upbound/upbound-aws-us-east-1/
[awg/ignore-empty-trailing//up]% up ctx upbound/upbound-aws-us-east-1/default/
Switched kubeconfig context to: Upbound upbound/upbound-aws-us-east-1/default/
[awg/ignore-empty-trailing//up]% up ctx upbound/upbound-aws-us-east-1///default///
Kubeconfig context "upbound": Upbound upbound/upbound-aws-us-east-1/default/
[awg/ignore-empty-trailing//up]% up ctx upbound/upbound-aws-us-east-1///default///test
Switched kubeconfig context to: Upbound upbound/upbound-aws-us-east-1/default/test
[awg/ignore-empty-trailing//up]% up ctx upbound/upbound-aws-us-east-1///default///test///
Kubeconfig context "upbound": Upbound upbound/upbound-aws-us-east-1/default/test