Assert non-empty assignments on kernelcreation

This yields better error messages

pystencils/gpucuda/kernelcreation.py

@@ -16,6 +16,7 @@ def create_cuda_kernel(assignments,
                        ghost_layers=None,
                        skip_independence_check=False,
                        use_textures_for_interpolation=True):
+    assert assignments, "Assignments must not be empty!"
     fields_read, fields_written, assignments = add_types(assignments, type_info, not skip_independence_check)
     all_fields = fields_read.union(fields_written)
     read_only_fields = set([f.name for f in fields_read - fields_written])

pystencils/kernelcreation.py

-from types import MappingProxyType
 from itertools import combinations
+from types import MappingProxyType
 
 import sympy as sp
 
@@ -74,6 +74,7 @@ def create_kernel(assignments,
                [0., 0., 0., 0., 0.]])
     """
     # ----  Normalizing parameters
+    assert assignments, "Assignments must not be empty!"
     split_groups = ()
     if isinstance(assignments, AssignmentCollection):
         if 'split_groups' in assignments.simplification_hints: