Fix #21256 - Empty hex string causes ICE

[dkorpel]

Closes #21256

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Testing this PR locally

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[dkorpel]

Hold on, string casting to scalar types is really funky...

void main()
{
    string s;
    if (s) {} // ok
    if ("abc") {} // ok
    auto x = cast(bool) s; // frontend error
    auto y = cast(bool) "abc"; // ok
    auto z = cast(ubyte) s; // frontend error
    auto w = cast(ubyte) "abc"; // e2ir error
}