Empty markdown out of bounds exception

[BernieWhite]

Description of the issue

When rule markdown help .md is an empty file, PSRule returns an exception.

Expected behaviour

PSRule should provide a more specific error or just not return any help.

Error output

Exception             :
    Type       : System.IndexOutOfRangeException
    TargetSite : Void .ctor(System.String)
    StackTrace :
   at PSRule.Parser.MarkdownStream..ctor(String markdown) in /home/vsts/work/1/s/src/PSRule/Parser/MarkdownStream.cs:line 97
   at PSRule.Parser.MarkdownReader.Read(String markdown, String path) in /home/vsts/work/1/s/src/PSRule/Parser/MarkdownReader.cs:line 82
   at PSRule.Commands.NewRuleDefinitionCommand.TryDocument(String path, RuleDocument& document) in /home/vsts/work/1/s/src/PSRule/Commands/NewRuleDefinitionCommand.cs:line 162
   at PSRule.Commands.NewRuleDefinitionCommand.GetHelpInfo(RunspaceContext context, String name) in /home/vsts/work/1/s/src/PSRule/Commands/NewRuleDefinitionCommand.cs:line 145
   at PSRule.Commands.NewRuleDefinitionCommand.ProcessRecord() in /home/vsts/work/1/s/src/PSRule/Commands/NewRuleDefinitionCommand.cs:line 75
   at System.Management.Automation.CommandProcessor.ProcessRecord()
    Message    : Index was outside the bounds of the array.
    Source     : PSRule
    HResult    : -2146233080
CategoryInfo          : NotSpecified: (:) [Invoke-PSRule], IndexOutOfRangeException
FullyQualifiedErrorId : System.IndexOutOfRangeException,Invoke-PSRule

Module in use and version:

• Module: PSRule
• Version: 0.19.0

Captured output from $PSVersionTable:

Name                           Value
----                           -----
PSVersion                      7.0.3
PSEdition                      Core
GitCommitId                    7.0.3
OS                             Microsoft Windows 10.0.19041
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0