fix: allow filtering for joins

...when selecting a source type in the alignment view.

Before no results would be shown because the internal check expects all
types to match while in the view only one can be specified.

common/plugins/eu.esdihumboldt.hale.common.align/src/eu/esdihumboldt/hale/common/align/model/impl/DefaultAlignment.java

@@ -419,9 +419,32 @@ public Collection<? extends Cell> getTypeCells(Cell queryCell) {
 					.getDefinition().getType();
 			if (target == null || DefinitionUtil.isSuperType(target, typeCellTarget)) {
 				// target matches
-				if (sources.isEmpty() || matchesSources(typeCell.getSource(), sources)) {
-					// source matches, too
-					result.add(typeCell);
+
+				if (sources.size() == 1) {
+					/*
+					 * Special case handling when there is exactly one type,
+					 * that in this case also allows to find a Join that only
+					 * uses one of the types.
+					 * 
+					 * Unclear if this is intended, but the else case does not
+					 * yield any results in such cases, which could be due to
+					 * the implementation of `matchesSources` in that case being
+					 * focused on property relations.
+					 */
+					TypeEntityDefinition type = sources.iterator().next();
+					if (sources.isEmpty() || matchesSources(type,
+							typeCell.getSource().values().stream()
+									.map(e -> AlignmentUtil.getTypeEntity(e.getDefinition()))
+									.toList())) {
+						// source matches, too
+						result.add(typeCell);
+					}
+				}
+				else {
+					if (sources.isEmpty() || matchesSources(typeCell.getSource(), sources)) {
+						// source matches, too
+						result.add(typeCell);
+					}
 				}
 			}
 		}