PairwiseMetric euclidean_distance(x,nil) should forward y = nil to squared_error

[gmenel]

Prevent double calculation and use x.transpose for performance.
squared_error needs to know if nil, in order to just transpose and not recalculate.
sum_y_vec = y.nil? ? sum_x_vec.transpose : ...

[yoshoku]

@dysonreturns
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