Get-PSRule generates exception when no .rule.ps1 scripts exist in path

[BernieWhite]

Description of the issue

Get-PSRule does not gracefully handle the case where no .rule.ps1 files exist in the path and raises exceptions.

To Reproduce

Steps to reproduce the issue:

# With no .rule.ps1 files in current and child directories
Get-PSRule -Verbose;

Expected behaviour

Get-PSRule should return no results and no exceptions.

Error output

VERBOSE: [Get-PSRule]::BEGIN
VERBOSE: Attempting to read: .\psrule.yml
VERBOSE: [PSRule][D] -- Scanning for source files: C:\dev\Workspace\PSRule\dummy
The property 'Length' cannot be found on this object. Verify that the property exists.
At C:\Users\nnn\Documents\WindowsPowerShell\Modules\PSRule\0.2.0\PSRule.psm1:265 char:57
+ ... Verbose -Message "[Get-PSRule] -- Found $($sourceFiles.Length) script ...
+                                               ~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], PropertyNotFoundException
    + FullyQualifiedErrorId : PropertyNotFoundStrict

VERBOSE: [Get-PSRule] -- Found  script(s)
Exception calling "Join" with "2" argument(s): "Value cannot be null.
Parameter name: values"
At C:\Users\nnn\Documents\WindowsPowerShell\Modules\PSRule\0.2.0\PSRule.psm1:266 char:64
+ ... [Get-PSRule] -- Found scripts: $([String]::Join(' ', $sourceFiles))";
+                                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : ArgumentNullException

Object reference not set to an instance of an object.
At C:\Users\nnn\Documents\WindowsPowerShell\Modules\PSRule\0.2.0\PSRule.psm1:286 char:13
+             $pipeline.Process();
+             ~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (:) [], NullReferenceException
    + FullyQualifiedErrorId : System.NullReferenceException```

**Module in use and version:**

- Module: PSRule
- Version: 0.1.0 and 0.2.0-B190105