[ENHANCEMENT] Have `op_term(::Tuple{Number})` return the identity

[jlbosse]

When programatically constructing OpSums I sometimes encounter terms that are simply proportional to the identity and would hence be represented by the empty tuple (if the prefactor is 1) or by a tuple just containing a number. Currently, constructing an op_term from such a tuple fails with a BoundsError as shown below:

julia> using ITensors

julia> ITensors.Ops.op_term((2.0, ))
ERROR: BoundsError: attempt to access 0-element Vector{Int64} at index [1]
Stacktrace:
 [1] throw_boundserror(A::Vector{Int64}, I::Tuple{Int64})
   @ Base ./essentials.jl:14
 [2] getindex(A::Vector{Int64}, i::Int64)
   @ Base ./essentials.jl:916
 [3] first
   @ ./abstractarray.jl:452 [inlined]
 [4] split(f::Function, t::Tuple{})
   @ ITensors.Ops ~/.julia/packages/ITensors/VGXV2/src/lib/Ops/src/op.jl:24
 [5] op_term(a::Tuple{})
   @ ITensors.Ops ~/.julia/packages/ITensors/VGXV2/src/lib/Ops/src/op.jl:262
 [6] op_term(a::Tuple{Float64})
   @ ITensors.Ops ~/.julia/packages/ITensors/VGXV2/src/lib/Ops/src/op.jl:245
 [7] top-level scope
   @ REPL[6]:1

The desired behaviour would be something along the lines of

julia> ITensors.Ops.op_term((2.0, ))
2.0 Id()

which would be consistent with the empty product being 1.

My current work-around is to explicitly pass in (2.0, "Id", site)

julia> ITensors.Ops.op_term((2.0, "Id", 1))
2.0 Id(1,)

but this requires writing an extra if noperators == 0 check in my code.