Fix index error when using Get-Command on Should (pester#1804)

src/functions/assertions/Should.ps1

@@ -119,8 +119,10 @@ function Should {
 
     dynamicparam {
         # Figuring out if we are using the old syntax is 'easy'
-        $myLine = # we can use $myInvocation.Line to get the surrounding context
+        # we can use $myInvocation.Line to get the surrounding context
+        $myLine = if ($null -ne $MyInvocation -and 0 -le ($MyInvocation.OffsetInLine - 1)) {
             $MyInvocation.Line.Substring($MyInvocation.OffsetInLine - 1)
+        }
 
         # A bit of Regex lets us know if the line used the old form
         if ($myLine -match '^\s{0,}should\s{1,}(?<Operator>[^\-\@\s]+)')

tst/functions/assertions/Should.Tests.ps1

@@ -118,4 +118,10 @@ InPesterModuleScope {
             { throw "abc" } | Should -ErrorId "abc" # -Throw is not used
         }
     }
+
+    Describe 'Using Should without MyInvocation when generating help' {
+        It 'Get-Command Should -Syntax should show all help' {
+            Get-Command Should -Syntax | Should -Not -BeNullOrEmpty
+        }
+    }
 }