Return null from StartsWithValueConverter on empty input

[jhaygood86]

Problem

StartsWithValueConverter.Transform skips leading whitespace and then reads Current without checking whether the enumerator was exhausted:

while (enumerator.MoveNext() && enumerator.Current.Type == TokenType.Whitespace)
{
    //Empty on purpose.
}

if (enumerator.Current.Type != _type || ...) return null;   // <-- enumerator may be finished

When the sequence is empty or whitespace-only, the loop exits because MoveNext() returned false, and Current is then either null or throws depending on the enumerator implementation:

Converters.IntegerConverter.StartsWithDelimiter().ConvertDefault();
// System.NullReferenceException

converter.Convert(new[] { Token.Whitespace });
// System.InvalidOperationException: Enumeration already finished.

Empty input isn't an exotic case here: ConvertDefault() passes Enumerable.Empty<Token>() by definition, and VaryStart falls back to ConvertDefault() once its token list is exhausted. Every other converter treats empty input as "no match" — DictionaryValueConverter, for instance, gets null back from ToIdentifier() and returns null.

Fix

Capture the MoveNext() result and return null when the sequence was exhausted.

This also unblocks composing StartsWithValueConverter into converters that get probed with empty input — for example a font-style: oblique <angle> converter, which is where I hit it.

Tests

5 tests in Property.cs:

• ConvertDefault() on a StartsWithDelimiter converter returns null (currently NullReferenceException)
• whitespace-only input returns null (currently InvalidOperationException)
• 3 guards over RatioConverter — the one composition that puts StartsWithDelimiter behind an ordered converter — parsing incomplete aspect-ratio media features

The first 2 fail on master; the 3 guards pass either way and are there to keep the real-world path covered. The full suite (1263 existing tests) stays green, and all seven target frameworks build with no new warnings.