Variant simple-type should allow 0-length sequence

[dvaliga]

Requires non-empty (length 1+) sequence. There are use cases where 0-length (empty) might be required.

[bmatern-nmdp]

Like this one:

        <consensus-sequence-block reference-sequence-id="ref11" start="8995" end="9961" phase-set="1" continuity="true">
                <sequence>
                    TGAGCTGGGAATCTGAGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGTGAGAGAGAGACAGAGAGACAGAGAGAGAGAGAGCGCCATCTGTGAGCATTTAGAATCCTCTCTATCCTGAGCAAGGAGTTCTGAGGGCACAGGTGTGTGTGTAGAGTGTGGATTTGTCTGTGTCTGTGAGGCTGTTGTGGGAGGGGAGGCAGGAGGGGGCTGCTTCTTATTCTTGGAGGACTCTGTGGGGAGGTGACAAGGGAGGTGGGTGCGGGCGGCTGGAGAGAGAGGTGACCTTGATTGTCTCGGGTCCTTAGAGATGCAGGGAAGGGAAATGTAAGGGGTGTGTGGTTGGGGTGAAGGTTTAGGGGAGGAGAGCTGAGGGGTAAGGAAGGTTTGGGATAATGTGAGGAGGCCAGTTCCAGACTGTCCCTGGCACACACCCTTCATGTAATCTCTGAAATAAAAGTGTGTGCTGTTTGTTTGTAAAAGCATTAGATTAATTTCTAGGGGAATTGAGGAGACCTCTGAGGCATCTCTGAAGCTTCTTTAGGTCTAAATTTCTTGCTAGTTTTTTGTTTTTTATTGTGTATATTTTTACATAGTAGAAATGACTGTGAAACTAACTTTTTGAATTAAAGTTTTAACACAGTTACTATTTTATTATAATGCTAATAGTTTTCTAGTAGTTACATATTATTCTTTTATATATAATAGTTGTGACACAACTTACCTCACTTTCCCCTTTGTTGACCTTTATTATGACATTCACCAAAATTTGAAAATGTATGTTTCTGGTTAATTTTTAATTTATATTTTTTTCATTTATAATTCTTTTGAATTATTTTGACCTATTTATTGGCCAGTTTTAATAACTGCTGTAAGAATTCCCTATTGTATTTGGTAGGGAATGGACAATGATCTACTGCCTAATATCTCGAGGGCTTAGTATTTTTCTCA
                </sequence>

                <variant reference-bases="TG" alternate-bases="" start="9026" end="9028" />
            </consensus-sequence-block>