For example, dirty checking was not right for the following case:

```
model.int_column = "+5"
model.float_column = "0.5E+1"
model.decimal_column = "0.5e-3"
```

It is enough to see whether leading character is a digit for avoiding
invalid numeric expression like 'wibble' to be type-casted to 0, as
this method's comment says.

Fixes #33801