Adds posting amount even when not given

[tbm]

2020-01-01 open Assets:Test

2020-07-24 * "Test"
  Assets:Test     100.00 EUR
  Assets:Test

becomes:

account Assets:Test

2020-07-24 * Test
  Assets:Test                                                    100.00 EUR
  Assets:Test                                                   -100.00 EUR

But -100.00 EUR was not specified, so it might be more idiomatic not to show it.