fix: curie validation in metamodelcore erroneous

CURIE validation in metamodelcore was erroneous and rejecting valid
CURIEs like those missing a prefix and a colon. This patch fixes it.

Signed-off-by: Silvano Cirujano Cuesta <silvano.cirujano-cuesta@siemens.com>

Author: Silvanoc

linkml_runtime/utils/metamodelcore.py

@@ -172,17 +172,17 @@ def __init__(self, v: str) -> None:
 
     @classmethod
     def ns_ln(cls, v: str) -> Optional[tuple[str, str]]:
-        # See if this is indeed a valid CURIE, ie, it can be split by a colon
+        if not validate_curie(v):
+            return None
+        # assume the presence of a colon and try to split the CURIE
         curie_split = v.split(":", 1)
         if len(curie_split) == 1:
-            # there is no ':' character in the string, ie, it is not a valid CURIE
-            return None
+            # there is no ':' character in the string, it's only a reference
+            return '', v
         else:
             prefix = curie_split[0].lower()
-            if not NCName.is_valid(prefix):
-                return None
             reference = curie_split[1]
-        return prefix, reference
+            return prefix, reference
 
     @classmethod
     def is_valid(cls, v: str) -> bool: