Anisotropic optical conductivity accompanied by a small energy gap in the one-dimensional thermoelectric telluride <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Ta</mml:mi><mml:mn>4</mml:mn></mml:msub><mml:msub><mml:mi>SiTe</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>