Add no cover for to_supported_dtypes()

Author: vlad-perevezentsev

dpnp/dpnp_iface_statistics.py

@@ -576,7 +576,7 @@ def correlate(a, v, mode="valid"):
     rdtype = result_type_for_device([a.dtype, v.dtype], device)
     supported_dtype = to_supported_dtypes(rdtype, supported_types, device)
 
-    if supported_dtype is None:
+    if supported_dtype is None:  # pragma: no cover
         raise ValueError(
             f"function does not support input types "
             f"({a.dtype.name}, {v.dtype.name}), "

dpnp/dpnp_utils/dpnp_utils_common.py

@@ -78,4 +78,4 @@ def is_castable(dtype, stype):
             ):
                 return stypes
 
-    return None
+    return None  # pragma: no cover