Allow for shell vars set to an empty value

[marckhouzam]

What this PR does / why we need it

If an shell variable is set it takes precedence over the same variable set in the tanzu config. However, if a shell variable was set to an empty value it was ignored.

With this commit, an empty value is no longer ignored as it is a valid value for a shell variable.

See #623 for an example.

Which issue(s) this PR fixes

Fixes #623

Describe testing done for PR

# Clean the plugin DB
$ rm ~/.cache/tanzu/plugin_inventory/default/

# Set a signature skip value for a test repo
$ tz config set env.TANZU_CLI_PLUGIN_DISCOVERY_IMAGE_SIGNATURE_VERIFICATION_SKIP_LIST localhost:9876/tanzu-cli/plugins/central:small

# Set a shell variable that should take precedence that says "do not SKIP anything" in this shell
$ export TANZU_CLI_PLUGIN_DISCOVERY_IMAGE_SIGNATURE_VERIFICATION_SKIP_LIST=''

# Confirm that the signature verification is performed, which means the shell variable is read
# and takes precedence over the one in the tanzu config 
$ tz plugin source update default -u localhost:9876/tanzu-cli/plugins/central:small
[i] Reading plugin inventory for "localhost:9876/tanzu-cli/plugins/central:small", this will take a few seconds.
[!] Unable to verify the plugins discovery image signature: failed validating the signature of the image localhost:9876/tanzu-cli/plugins/central:small :no matching signatures:
invalid signature when validating ASN.1 encoded signature
[x] Fatal, plugins discovery image signature verification failed. The `tanzu` CLI can not ensure the integrity of the plugins to be installed. To ignore this validation please append "localhost:9876/tanzu-cli/plugins/central:small" to the comma-separated list in the environment variable "TANZU_CLI_PLUGIN_DISCOVERY_IMAGE_SIGNATURE_VERIFICATION_SKIP_LIST".  This is NOT RECOMMENDED and could put your environment at risk!

# Unset the shell variable and see that the one from the tanzu config is then read and that a skip is done
$ unset TANZU_CLI_PLUGIN_DISCOVERY_IMAGE_SIGNATURE_VERIFICATION_SKIP_LIST
$ tz plugin source update default -u localhost:9876/tanzu-cli/plugins/central:small
[i] Reading plugin inventory for "localhost:9876/tanzu-cli/plugins/central:small", this will take a few seconds.
[!] Skipping the plugins discovery image signature verification for "localhost:9876/tanzu-cli/plugins/central:small"

[ok] updated discovery source default

Release note

A shell variable set to an empty value is no longer ignored by the CLI and takes precedence over a similar variable present in the tanzu config, as expected.

Additional information

Special notes for your reviewer

[anujc25]

LGTM