Allow empty UoM (compare MaterialDemand)

io.catenax.capacity_group/1.0.0/CapacityGroup.ttl

@@ -95,7 +95,7 @@
     bamm:description "The Enumeration lists a selection of allowed units of measure referenced by their official code according to Codes for Units of Measure Used in International Trade"@en;
     bamm:see <https://tfig.unece.org/contents/recommendation-20.htm>;
     bamm:dataType xsd:string;
-    bamm-c:values ("GRM" "KGM" "TNE" "STN" "ONZ" "LBR" "CMT" "MTR" "KTM" "INH" "FOT" "YRD" "CMK" "MTK" "INK" "FTK" "YDK" "CMQ" "MTQ" "INQ" "FTQ" "YDQ" "MLT" "LTR" "HLT" "H87" "SET" "PR" "ZP" "KWH").
+    bamm-c:values ("GRM" "KGM" "TNE" "STN" "ONZ" "LBR" "CMT" "MTR" "KTM" "INH" "FOT" "YRD" "CMK" "MTK" "INK" "FTK" "YDK" "CMQ" "MTQ" "INQ" "FTQ" "YDQ" "MLT" "LTR" "HLT" "H87" "SET" "PR" "ZP" "KWH" "").
 :LinkedDemandSeriesSet a bamm-c:Set;
     bamm:preferredName "Linked Demand Series Set"@en;
     bamm:description "A set of demand series assigned to a certain capacity group."@en;