Why replacing null values with outliers?
-999999 is pretty common when you work with trees or forests (you imputatte missing values with the value that differs much from other values to perform better splitting).

Maybe (!) in some rare cases imputation by -999999 may be useful (for example, I used it by clustering) though.

Mean/median as advised are really good-working approaches. Including, for example, boolean column that indicates missing values (=1; before imputation) and non-missing (=0).