test: verify non-existing function invocation

Yields `null`.

Closes #539

TestCases/compliance-level-3/1131-feel-function-invocation/1131-feel-function-invocation-test-01.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<!-- Contributed to DMN TCK by nikku (https://github.com/nikku) -->
+<testCases xmlns="http://www.omg.org/spec/DMN/20160719/testcase" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
+    <modelName>1131-feel-function-invocation.dmn</modelName>
+    <labels>
+        <label>Compliance Level 3</label>
+        <label>Data Type: Function</label>
+    </labels>
+    <testCase id="001">
+        <description>Non existing function evaluates to null (10.3.2.13.2)</description>
+        <resultNode errorResult="true" name="decision001" type="decision">
+            <expected>
+                <value xsi:nil="true"/>
+            </expected>
+        </resultNode>
+    </testCase>
+</testCases>

TestCases/compliance-level-3/1131-feel-function-invocation/1131-feel-function-invocation.dmn

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<definitions namespace="http://www.montera.com.au/spec/DMN/1131-feel-function-invocation" name="1131-feel-function-invocation" id="_i9fboPUasda" xmlns="https://www.omg.org/spec/DMN/20191111/MODEL/" xmlns:di="http://www.omg.org/spec/DMN/20180521/DI/" xmlns:dmndi="https://www.omg.org/spec/DMN/20191111/DMNDI/" xmlns:dc="http://www.omg.org/spec/DMN/20180521/DC/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+    <description>FEEL invocation</description>
+    <decision name="decision001" id="_decision001">
+        <description>Tests invocation of non-existing function evaluates to null (10.3.2.13.2)</description>
+        <variable name="decision_001"/>
+        <literalExpression>
+            <text>non_existing_function()</text>
+        </literalExpression>
+    </decision>
+</definitions>
+