Bug 431159

KMyMoney Version:
```
kmymoney --version
kmymoney 5.1.0
```

**Steps to reproduce:**

1) Account -> New Account
2) Choose a bank
3) Choose account type: Load (or credit)
4) Type: Take a loan
   Frequency: monthly
   interest change rate: monthly
   fixed interest
5) loan amount: empty
   interest rate: default 0
   duration: default 0
   amortization: empty
   rest (one time rate): 0,00

**Expected results:**

Insert a loan amount and click "calculate" to calculate the amortization. Delete the amortization again and click "calculate" again which will add the same amortization value.

**Actual results:**

Insert a loan amount and click "calculate" to calculate the amortization. The amortization is filled in, but can not be deleted. Removing the value and changing the focus will immediately enter 0,00 into that field.

Additionally it's not possible to recalculate the loan amount (will be filled with 0,00). And it's not possible to calculate the interest rate, duration or rest (one time rate) because these fields have already values and it's not possible to empty these fields.