do not export an empty unit attribute to ProB

de.prob.core/src/de/prob/eventb/translator/ContextTranslator.java

@@ -139,9 +139,12 @@ public final class ContextTranslator extends AbstractComponentTranslator {
 
 		for (final ISCConstant constant : constants) {
 			if (constant.hasAttribute(UNITATTRIBUTE)) {
+				String content = constant.getAttributeValue(UNITATTRIBUTE);
+
+				if (!content.isEmpty()) {
 					pragmas.add(new UnitPragma(getResource(), constant
-						.getIdentifierString(), constant
-						.getAttributeValue(UNITATTRIBUTE)));
+							.getIdentifierString(), content));
+				}
 			}
 		}
 	}

de.prob.core/src/de/prob/eventb/translator/internal/ModelTranslator.java

@@ -190,9 +190,12 @@ public class ModelTranslator extends AbstractComponentTranslator {
 
 		for (final IVariable variable : variables) {
 			if (variable.hasAttribute(UNITATTRIBUTE)) {
+				String content = variable.getAttributeValue(UNITATTRIBUTE);
+
+				if (!content.isEmpty()) {
 					pragmas.add(new UnitPragma(getResource(), variable
-						.getIdentifierString(), variable
-						.getAttributeValue(UNITATTRIBUTE)));
+							.getIdentifierString(), content));
+				}
 			}
 		}
 	}