Fix default template engine not loading Razor/Liquid processors

[linckez]

Fix Razor/Liquid template extraction not running by default

After the migration to CommandLineUtils, template.Value() returns null when -t isn't specified, but TemplateEngine.Both was string.Empty. Since null != "", the Razor and Liquid processors never got added unless -t was explicitly passed.

Fixed TemplateEngine.Both to be null instead of string.Empty.

Reproduce:

Create a Razor file with localizable strings, e.g.:

@inject IStringLocalizer<Home> S
<p>@S["Welcome to MudBlazor"]</p>
<p>@S["This is a test of string extraction."]</p>
<p>@S.Plural(5,"You have {0} notification.", "You have {0} notifications.", 6)</p>
<p>@S["Hello {0}, you have {1} notifications.", "User", 5]</p>

# before fix
dotnet run --project src/OrchardCoreContrib.PoExtractor -- /path/to/project /tmp/output
# shows: Found 0 strings.

# after fix
dotnet run --project src/OrchardCoreContrib.PoExtractor -- /path/to/project /tmp/output
# shows: Found 4 strings.

Confirmed working on v1.2.0 (d85eda7) as a reference.

[hishamco]

@linckez, could you please add a unit test?

[hishamco]

Why is this closed? If you need help with the unit testing, let me know