fix(scoop-import): Use foreach instead of ForEach-Object for nullity check

[zStruCat]

Description

Check whether the wanted property of bucket is null.
Check if the to-be-imported bucket is already added locally

Context

Closes #5033

[rashil2000]

I've made some tweaks:

• Using foreach loop in place of ForEach-Object automatically takes care of null values
• The add_bucket function has a robust check for duplicate bucket names and origin URIs, so we don't need to add another check. Moreover, if a bucket already exists, we want to show an error while importing, so that users know that bucket is there.

[niheaven]

LGTM

[zStruCat]

The add_bucket function has a robust check for duplicate bucket names and origin URIs, so we don't need to add another check. Moreover, if a bucket already exists, we want to show an error while importing, so that users know that bucket is there.

I agree that "if a bucket already exists, we want to show an error while importing", but currently the resulted warning and hint is misleading in this context, especially:
use "scoop bucket rm main" to remove it
"'$($Item.Name)' is already in your local bucket. It won't be added again." is more specific and user-friendly in the context of using a import function.

[zStruCat]

I've changed accordingly.