bug: get_cost_pair should throw an error or return empty or a more informative error

[pesap]

This happens because this function fails when the vals object is empty

  last_increasing_point = findfirst(x -> x < 0.0, [diff(getfield.(vals, :x))..., -Inf])

Stack trace

ERROR: BoundsError: attempt to access 0-element Vector{Any} at index [1:1]
Stacktrace:
  [1] throw_boundserror(A::Vector{Any}, I::Tuple{UnitRange{Int64}})
    @ Base ./abstractarray.jl:737
  [2] checkbounds
    @ ./abstractarray.jl:702 [inlined]
  [3] getindex(A::Vector{Any}, I::UnitRange{Int64})
    @ Base ./array.jl:973
  [4] get_cost_pairs(gen::@NamedTuple{…}, cost_colnames::PowerSystems._HeatRateColumns)
    @ PowerSystems ~/dev/Sienna/PowerSystems.jl/src/parsers/power_system_table_data.jl:948
  [5] make_cost(::Type{…}, data::PowerSystemTableData, gen::@NamedTuple{…}, cost_colnames::PowerSystems._HeatRateColumns)
    @ PowerSystems ~/dev/Sienna/PowerSystems.jl/src/parsers/power_system_table_data.jl:833
  [6] make_thermal_generator(data::PowerSystemTableData, gen::@NamedTuple{…}, cost_colnames::PowerSystems._HeatRateColumns, bus::ACBus)
    @ PowerSystems ~/dev/Sienna/PowerSystems.jl/src/parsers/power_system_table_data.jl:1132
  [7] make_generator(data::PowerSystemTableData, gen::@NamedTuple{…}, cost_colnames::PowerSystems._HeatRateColumns, bus::ACBus, gen_storage::Tuple{…})
    @ PowerSystems ~/dev/Sienna/PowerSystems.jl/src/parsers/power_system_table_data.jl:796
  [8] gen_csv_parser!(sys::System, data::PowerSystemTableData)
    @ PowerSystems ~/dev/Sienna/PowerSystems.jl/src/parsers/power_system_table_data.jl:563
  [9] System(data::PowerSystemTableData; time_series_resolution::Nothing, time_series_in_memory::Bool, time_series_directory::Nothing, runchecks::Bool, kwargs::@Kwargs{})
    @ PowerSystems ~/dev/Sienna/PowerSystems.jl/src/parsers/power_system_table_data.jl:279
 [10] top-level scope
    @ REPL[11]:1
Some type information was truncated. Use `show(err)` to see complete types.