Verify UTF8 is detected encoding of empty buffer

src/jes/core/encoding.d

@@ -302,7 +302,11 @@ struct Encoding
         import std.file;
         const(ubyte)[] dat;
         Encoding e;
-        size_t bom_size;
+        size_t bom_size = 54321;
+
+        e = Encoding.detect_encoding(null, 0, &bom_size);
+        assert(e == Encoding.UTF8);
+        assert(bom_size == 0u);
 
         dat = cast(const(ubyte)[])std.file.read("test/files/encodings/utf-8.txt");
         e = Encoding.detect_encoding(dat.ptr, dat.length, &bom_size);