Inference check for `exp`

[ytdHuang]

The matrix exponential has type inference issue.

using Test, QuantumToolbox
@inferred exp(-1.0im * sigmax())

ERROR message:

return type 
QuantumObject{SparseMatrixCSC{ComplexF64, Int64}, OperatorQuantumObject} 
does not match inferred return type 
Union{QuantumObject{Diagonal{ComplexF64, Vector{ComplexF64}}, OperatorQuantumObject}, QuantumObject{SparseMatrixCSC{ComplexF64, Int64}, OperatorQuantumObject}}

I believe this issue originates from this line: https://github.com/qutip/QuantumToolbox.jl/blob/main/src/qobj/arithmetic_and_attributes.jl#L400

If the input operator is an "empty" sparse matrix, the returned identity operator is in the type of Diagonal matrix.
We should convert it to SparseMatrixCSC in the inner function _spexp.