Empty sequence adds an undesired block

[beorostica]

When an empty sequence is created, it adds an unnecessary 1-block. Here is a MWE where I additionaly remove the undesired block:

julia> using KomaMRI

julia> seq = Sequence()
Sequence[ τ = 0.0 ms | blocks: 1 | ADC: 0 | GR: 0 | RF: 0 | DEF: 0 ]

julia> seq += Sequence([Grad(1,1)])
Sequence[ τ = 1000.0 ms | blocks: 2 | ADC: 0 | GR: 1 | RF: 0 | DEF: 0 ]

julia> seq = seq[2:end]
Sequence[ τ = 1000.0 ms | blocks: 1 | ADC: 0 | GR: 1 | RF: 0 | DEF: 0 ]

This should't have implications in the simulations, however is not the behaviour we expect.

[cncastillo]

This could fix it

Sequence() = Sequence(Matrix{Grad}(undef, 3, 0))

or defne

struct EmptySequence end
Sequence() = EmptySequence()
+(::EmptySequence, seq::Sequence) = ...
+(::EmptySequence, seq::RF) = ...
+(::EmptySequence, seq::Grad) = ...
+(::EmptySequence, seq::ADC) = ...