missing product validation leads to NPE

[madskaddie]

calling:

 entrybuilder.setProduct(productUID=null)

raises:

java.lang.NullPointerException: The field field.product can not be null
    at org.apache.commons.lang3.Validate.notNull(Validate.java:222)
    at com.premiumminds.billy.core.util.BillyValidator.notNull(BillyValidator.java:88)

while calling entrybuilder.build() without having called setProduct() raises:

java.lang.NullPointerException
    at com.premiumminds.billy.core.services.builders.impl.GenericInvoiceEntryBuilderImpl.validateValues(GenericInvoiceEntryBuilderImpl.java:294)
    at com.premiumminds.billy.core.services.builders.impl.GenericInvoiceEntryBuilderImpl.validateInstance(GenericInvoiceEntryBuilderImpl.java:265)
    at com.premiumminds.billy.portugal.services.builders.impl.PTGenericInvoiceEntryBuilderImpl.validateInstance(PTGenericInvoiceEntryBuilderImpl.java:81)
    at com.premiumminds.billy.portugal.services.builders.impl.PTInvoiceEntryBuilderImpl.validateInstance(PTInvoiceEntryBuilderImpl.java:60)
    at com.premiumminds.billy.core.services.builders.impl.AbstractBuilder.build(AbstractBuilder.java:65)
    at com.premiumminds.billy.core.services.builders.impl.GenericInvoiceBuilderImpl.addEntry(GenericInvoiceBuilderImpl.java:239)
    at com.premiumminds.billy.core.util.NotOnUpdateInterceptor.invoke(NotOnUpdateInterceptor.java:54)
    at pt.ggb.sanbox.billy.InvoiceCreation.testCreate(InvoiceCreation.java:58)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
    at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:30)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)