fix(bucket): Return empty list correctly in Get-LocalBucket

[lewis-yeung]

Description

1. Return a zero-length list instead of $null in Get-LocalBucket.

2. Adjust the output message for scoop alias list when there is no alias found.

3. Adjust the output message for scoop cache show when there is no cache found.

Motivation and Context

1. I ran into an error when I removed all buckets and was trying to add them back using URLs of mirrors, which means running scoop bucket list when there is nothing in $SCOOP\buckets:

   Get-Item: <path_to_scoop>\apps\scoop\current\lib\buckets.ps1:134:32
   Line |
    134 |              $bucket.Updated = (Get-Item "$path\bucket").LastWriteTime
        |                                 ~~~~~~~~~~~~~~~~~~~~~~~
        | Cannot find path '<path_to_scoop>\buckets\main\bucket' because it does not exist.
   

   The Get-LocalBucket returns $null in this case, so Find-BucketDirectory gets the default value main for $Name:

   Scoop/lib/buckets.ps1

   Lines 126 to 128 in e6d0371

   	Get-LocalBucket | ForEach-Object {
   	$bucket = [Ordered]@{ Name = $_ }
   	$path = Find-BucketDirectory $_ -Root

   Scoop/lib/buckets.ps1

   Lines 13 to 14 in e6d0371

   	param(
   	[string] $Name = 'main',

   To fix this, we should use a zero-length list instead and give a tip of adding the default main bucket.

2. No additional description.

3. scoop cache show shows the following message when there is no cache found:

   Total: 0 files,  B
   

   I think it's better to change it to something like this:

   No cache found.
   

How Has This Been Tested?

Just run as usual:

scoop bucket list
scoop alias list
scoop cache show

Checklist:

• I have read the Contributing Guide.

[lewis-yeung]

@rashil2000 Changed and re-pushed. Please take a look. Thanks for your patience.