F grpc null values

Summary

Fixes the NullPointerException which was throws when executing SELECT queries on NULL Values, e.g. SELECT p1 FROM ENTITY WHERE p1 IS NULL. The implementation needs to be slightly different compared to SELECT p1 FROM ENTITY WHICH DOESN'T HAVE A p1 but the results must be the same.

Related: #311 (closed)

Focus

Actually there are three fixed bugs in this MR. The fix of the first revealed the second, which revealed the third. See annotations.

Test Environment

No manual testing required.

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src/main/java/org/caosdb/server/entity/Entity.java

@@ -609,10 +609,11 @@ public abstract class Entity extends AbstractObservable implements EntityInterfa
 
   @Override
   public void parseValue() throws Message {
-    if (!this.isParsed) {
-      this.isParsed = true;
-      setValue(getDatatype().parseValue(getValue()));
+    if (this.isParsed) {
+      return;
     }
+    this.isParsed = true;
+    if (getValue() != null) setValue(getDatatype().parseValue(getValue()));
   }
 
   @Override