Fixes #190

Problem

UnmarshalText([]byte("")) returns a bare io.EOF error:

var d decimal.Big
err := d.UnmarshalText([]byte(""))
// err == io.EOF ← confusing, looks like a Reader was consumed

This is confusing in JSON unmarshaling contexts where io.EOF is typically interpreted as the input stream being exhausted, not as invalid input.

Fix

In scanSign, convert io.EOF to a descriptive error: "decimal: cannot scan empty string".

var d decimal.Big
err := d.UnmarshalText([]byte(""))
// err == "decimal: cannot scan empty string" ← clear