Fix/div by zero (#5041)

* Handle gas_price being zero separately

* Bump spec_version

* Add a unit & integration tests for gas price = 0

substrate/bin/node/runtime/src/lib.rs

@@ -81,8 +81,8 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to 0. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 225,
-	impl_version: 1,
+	spec_version: 226,
+	impl_version: 0,
 	apis: RUNTIME_API_VERSIONS,
 };