Update src/Bootstrap/Views/Shared/_validationSummary.cshtml

Found a bug where the alert div was being rendered when the only validation error found is not bound to a property and exclude properties is true. This is because ValidationSummary pushes in a blank entry in this scenario.
Merge pull request #76 from roryprimrose/patch-2

Update src/Bootstrap/Views/Shared/_validationSummary.cshtml