Table.SortBy can't handle empty cells

Describe the bug

The last column of your test table has no values for two rows. Sorting that column fails.

To Reproduce

Put this into table_test.go:

func TestTable_MoreSort(t *testing.T) {
    table := Table{}
    table.AppendRows(testRows)
    table.AppendHeader(testHeader)
    table.SetStyle(styleTest)
    expectedOut := `(-----^------------^-----------^--------^-----------------------------)
[<#  >|<FIRST NAME>|<LAST NAME>|<SALARY>|<                           >]
{-----+------------+-----------+--------+-----------------------------}
[<20 >|<Jon       >|<Snow     >|<  2000>|<You know nothing, Jon Snow!>]
[<300>|<Tyrion    >|<Lannister>|<  5000>|<                           >]
[<1  >|<Arya      >|<Stark    >|<  3000>|<                           >]
\-----v------------v-----------v--------v-----------------------------/`
    table.SortBy([]SortBy{{Number: 5, Mode: Dsc}, {Name: "Last Name", Mode: Asc}})
    assert.Equal(t, expectedOut, table.Render())
}

Expected behavior

Expactation is that empty cells are sorted according to their string, representation.

The expected output is given in the code.

Screenshots

Actual output:

(-----^------------^-----------^--------^-----------------------------)
[<  #>|<FIRST NAME>|<LAST NAME>|<SALARY>|<                           >]
{-----+------------+-----------+--------+-----------------------------}
[<300>|<Tyrion    >|<Lannister>|<  5000>|<                           >]
[< 20>|<Jon       >|<Snow     >|<  2000>|<You know nothing, Jon Snow!>]
[<  1>|<Arya      >|<Stark    >|<  3000>|<                           >]
\-----v------------v-----------v--------v-----------------------------/

As you see: Column 5 isn't properly sorted.

Software (please complete the following information):

OS: Mac OS - But I'm using a dockerized go
GoLang Version dockerized golang:latest

jedib0t / go-pretty

Table.SortBy can't handle empty cells #305