Allow empty partial-block

[TheConstructor]

I authored a partial-template with optional block-includes and happened to write {{#>mail}}{{/mail}} for the test-mail, as I did not need to adjust the optionals. This led to an exception similar to the one below:

System.InvalidOperationException
Sequence contains no elements
   at System.Linq.ThrowHelper.ThrowNoElementsException()
   at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)
   at HandlebarsDotNet.Compiler.PartialBlockAccumulatorContext.GetAccumulatedBlock() in ..\Handlebars.Net\source\Handlebars\Compiler\Lexer\Converter\BlockAccumulators\PartialBlockAccumulatorContext.cs:line 27
   at HandlebarsDotNet.Compiler.BlockAccumulator.AccumulateBlock(Expression parentItem, IEnumerator`1 enumerator, BlockAccumulatorContext context) in ..\Handlebars.Net\source\Handlebars\Compiler\Lexer\Converter\BlockAccumulator.cs:line 56
   at HandlebarsDotNet.Compiler.BlockAccumulator.ConvertTokens(IEnumerable`1 sequence)+MoveNext() in ..\Handlebars.Net\source\Handlebars\Compiler\Lexer\Converter\BlockAccumulator.cs:line 32
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at HandlebarsDotNet.Compiler.BlockAccumulator.Accumulate(IEnumerable`1 tokens, ICompiledHandlebarsConfiguration configuration) in ..\Handlebars.Net\source\Handlebars\Compiler\Lexer\Converter\BlockAccumulator.cs:line 13
   at HandlebarsDotNet.Compiler.ExpressionBuilder.ConvertTokensToExpressions(IEnumerable`1 tokens, ICompiledHandlebarsConfiguration configuration) in ..\Handlebars.Net\source\Handlebars\Compiler\ExpressionBuilder.cs:line 25
   at HandlebarsDotNet.Compiler.HandlebarsCompiler.Compile(ExtendedStringReader source, CompilationContext compilationContext) in ..\Handlebars.Net\source\Handlebars\Compiler\HandlebarsCompiler.cs:line 25
   at HandlebarsDotNet.HandlebarsEnvironment.Compile(TextReader template) in ..\Handlebars.Net\source\Handlebars\HandlebarsEnvironment.cs:line 145
   at HandlebarsDotNet.HandlebarsEnvironment.Compile(String template) in ..\Handlebars.Net\source\Handlebars\HandlebarsEnvironment.cs:line 188
   at HandlebarsDotNet.Handlebars.Compile(String template) in ..\Handlebars.Net\source\Handlebars\Handlebars.cs:line 63
   at HandlebarsDotNet.Test.InlinePartialTests.BasicBlockInlinePartial() in ..

Being new to Handlebars.Net it took me a while to understand what really was wrong. I no know, that I can change the template to {{>mail}}, but the exception isn't really helpful and I don't know why it shouldn't just render. So this pull-request allows block-includes with empty blocks.

On a side-note {{#>mail}}{{/mail}} renders, if no mail-partial is present, while {{>mail}} throws an exception, if no MissingPartialTemplateHandler is registered. While not necessary for my use-case it may even be nice to have a syntax, that allows for optional partials out-of-the-box.