Fix the checksum type check

It incorrectly returned valid for a `list` of alternative checksums
while it must be a `tuple`. The `None` case was missed and due to the
unrestricted `tuple` elem_type it may return valid for actually invalid
entries. So restrict to tuples of strings which are valid at that point
but that may wrongly return invalid. But in that case the conversion
function will be called which does more elaborate verification for those
cases.

easybuild/framework/easyconfig/types.py

@@ -563,19 +563,28 @@ def ensure_iterable_license_specs(specs):
 }))
 # checksums is a list of checksums, one entry per file (source/patch)
 # each entry can be:
+# None
 # a single checksum value (string)
 # a single checksum value of a specified type (2-tuple, 1st element is checksum type, 2nd element is checksum)
-# a list of checksums (of different types, perhaps different formats), which should *all* be valid
-# a dictionary with a mapping from filename to checksum value
-CHECKSUM_LIST = (list, as_hashable({'elem_types': [str, tuple, STRING_DICT]}))
-CHECKSUMS = (list, as_hashable({'elem_types': [str, tuple, STRING_DICT, CHECKSUM_LIST]}))
+# a tuple of checksums (of different types, perhaps different formats), which should *all* be valid
+# a dictionary with a mapping from filename to checksum (None, value, type&value, alternatives)
+#
+# For simplicity we only allow the case(s) where the tuple consists only of strings
+# and leave further verification to the to_checksums function
+CHECKSUM_DICT = (dict, as_hashable(
+    {
+        'elem_types': [type(None), str, TUPLE_OF_STRINGS],
+        'key_types': [str],
+    }
+))
+CHECKSUMS = (list, as_hashable({'elem_types': [type(None), str, TUPLE_OF_STRINGS, CHECKSUM_DICT]}))
 
-CHECKABLE_TYPES = [CHECKSUM_LIST, CHECKSUMS, DEPENDENCIES, DEPENDENCY_DICT, LIST_OF_STRINGS,
+CHECKABLE_TYPES = [CHECKSUM_DICT, CHECKSUMS, DEPENDENCIES, DEPENDENCY_DICT, LIST_OF_STRINGS,
                    SANITY_CHECK_PATHS_DICT, STRING_DICT, STRING_OR_TUPLE_LIST, STRING_OR_TUPLE_OR_DICT_LIST,
                    TOOLCHAIN_DICT, TUPLE_OF_STRINGS]
 
 # easy types, that can be verified with isinstance
-EASY_TYPES = [string_type, bool, dict, int, list, str, tuple]
+EASY_TYPES = [string_type, bool, dict, int, list, str, tuple, type(None)]
 
 # type checking is skipped for easyconfig parameters names not listed in PARAMETER_TYPES
 PARAMETER_TYPES = {