[ModelicaSystem] split __init__() - mypy fix in ModelicaSystemCmd

Author: syntron

OMPython/ModelicaSystem.py

@@ -112,7 +112,15 @@ def __getitem__(self, index: int):
 class ModelicaSystemCmd:
     """A compiled model executable."""
 
-    def __init__(self, runpath: OMCPath, modelname: str, timeout: Optional[float] = None) -> None:
+    def __init__(
+            self,
+            runpath: OMCPath,
+            modelname: Optional[str] = None,
+            timeout: Optional[float] = None,
+    ) -> None:
+        if modelname is None:
+            raise ModelicaSystemError("Missing model name!")
+
         self._runpath = runpath
         self._model_name = modelname
         self._timeout = timeout