Are there exercises which strengthens the abdominal wall and which can help against developing a hernia in first place?

No guarantee : Core strengthening exercises as practiced in pillates and yogs will generally strengthen the abdominal wall without exacerbating existing hernias if performed correctly. Any other exercises which require you to bear down and create excessive intra-abdominal pressure are more likely to aggravate an existing hernia.

No: Many people with strong abdominal muscles get hernias, and many people with weaker muscles don't have hernias. There is most likely a genetic predisposition to hernia formation based on the types of connective tissue that any given individual has. Keep exercising though, its great for lots of other things!