Adding check for empty string as well as null (#102)

When using the -Credentials parameter when calling `Invoke-PSDepend` for
dependencies that don't require credentials, would result in the
erroneous warning `WARNING: No credential found for the specified name .
Was the dependency misconfigured?`

### Example

```sh
Invoke-PSDepend -Path "./depends.psd1" -Install -Import -Credentials @{
  "repo1WithCredential" = (Get-Credential)
}
```

#### File `depends.psd1`
```sh
@{
    "PSModule1" = @{
        
        "Name"           = "PSModule1"
        "Version"        = "1.0.0"
        "DependencyType" = "PSGalleryModule"
        "Credential"     = "repo1WithCredential"
        "Parameters"     = @{
            "Repository" = "repo1WithCredential"
        }
    }
    "PSModule2" = @{
        "Name"           = "PSModule2"
        "Version"        = "2.1.0"
        "DependencyType" = "PSGalleryModule"
        "Parameters"     = @{
            "Repository" = "repo2WithoutCredential"
        }
```

Would generate the warning when processing the `PSModule2` dependency.

```sh
WARNING: No credential found for the specified name . Was the dependency misconfigured?
```

### Fix

The -Name parameter in `Resolve-Credential` is cast from $null to an
empty string "". So now both null and an empty string are checked.

---------

Signed-off-by: Gilbert Sanchez <me@gilbertsanchez.com>
Co-authored-by: rmiller@nordex-online.com <rmiller@nordex-online.com>
Co-authored-by: Gilbert Sanchez <me@gilbertsanchez.com>

Author: 3 people

PSDepend/Public/Get-Dependency.ps1

@@ -383,7 +383,7 @@ function Get-Dependency {
                 }
             }
         }
-    }
+	}
 
     # Helper to retrieve the credential for a dependency
     function Resolve-Credential {
@@ -393,7 +393,7 @@ function Get-Dependency {
         )
 
         $credential = $null
-        if (($null -ne $Name) -and ($null -ne $Credentials)) {
+        if (![string]::IsNullOrEmpty($Name) -and ($null -ne $Credentials)) {
 
             if ($Credentials.ContainsKey($Name)) {
                 $credential = $Credentials[$Name]