Bugfix: mock existing local function.

The mock was not applicable in case the function already exists and the specified closure is null.

src/main/groovy/com/lesfurets/jenkins/unit/InterceptingGCL.groovy

@@ -18,7 +18,7 @@ class InterceptingGCL extends GroovyClassLoader {
             Map.Entry<MethodSignature, Closure> matchingMethod = helper.allowedMethodCallbacks.find { k, v -> k == signature }
             if (matchingMethod) {
                 // a matching method was registered, replace script method execution call with the registered closure (mock)
-                metaClazz."$scriptMethod.name" = matchingMethod.value
+                metaClazz."$scriptMethod.name" = matchingMethod.value ?: {}
             }
         }
     }

src/test/groovy/com/lesfurets/jenkins/unit/declarative/TestMockLocalFunction.groovy

@@ -0,0 +1,26 @@
+package com.lesfurets.jenkins.unit.declarative
+
+import org.junit.Before
+import org.junit.Test
+
+class TestMockLocalFunction extends DeclarativePipelineTest {
+
+    @Before
+    @Override
+    void setUp() throws Exception {
+        scriptRoots += 'src/test/jenkins/jenkinsfiles'
+        helper.registerAllowedMethod("githubNotify", [Map])
+        super.setUp()
+    }
+
+    @Test(expected=MissingMethodException.class)
+    void should_execute_with_errors() {
+        runScript("Mock_existing_function_Jenkinsfile")
+    }
+
+    @Test
+    void should_execute_without_errors() {
+        helper.registerAllowedMethod("runFunc")
+        runScript("Mock_existing_function_Jenkinsfile")
+    }
+}

src/test/jenkins/jenkinsfiles/Mock_existing_function_Jenkinsfile

@@ -0,0 +1,16 @@
+pipeline {
+    agent { node { label 'test1' } }
+    stages {
+        stage('sdfsdf') {
+            steps {
+                githubNotify status: 'PENDING', context: 'Pipeline'
+                runFunc()
+            }
+        }
+    }
+}
+
+def runFunc(){
+    echo "inside runFunc"
+    doSomeStuff()
+}