Do not use Option to wrap GenesisConfig fields (#8275)

Currently we wrap every `GenesisConfig` field in an `Option`, while
we require `Default` being implemented for all pallet genesisconfigs.
Passing `None` also results in the genesis not being initialized, which
is a bug as seen from the perspective of a pallet developer?

This pr changes the fields of the `GenesisConfig` to non `Option` types.

substrate/frame/example/src/lib.rs

@@ -814,14 +814,14 @@ mod tests {
 	pub fn new_test_ext() -> sp_io::TestExternalities {
 		let t = GenesisConfig {
 			// We use default for brevity, but you can configure as desired if needed.
-			frame_system: Some(Default::default()),
-			pallet_balances: Some(Default::default()),
-			pallet_example: Some(pallet_example::GenesisConfig {
+			frame_system: Default::default(),
+			pallet_balances: Default::default(),
+			pallet_example: pallet_example::GenesisConfig {
 				dummy: 42,
 				// we configure the map with (key, value) pairs.
 				bar: vec![(1, 2), (2, 3)],
 				foo: 24,
-			}),
+			},
 		}.build_storage().unwrap();
 		t.into()
 	}