C#: Add a tests for MaybeNullExpr.

Author: michaelnebel

csharp/ql/test/library-tests/dataflow/nullness/MaybeNullExpr.cs

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+using System;
+
+public class C
+{
+    string Prop { get; set; }
+
+    void M(object o, bool b)
+    {
+        // Conditional expr might be null.
+        var conditionalExpr = b ? new object() : null;
+
+        // Null-coalescing expr might be null as the right operand is null.
+        var nullCoalescing = o ?? null;
+
+        // Cast might be null.
+        var c = o as C;
+
+        // Conditional access might be null as the qualifier might be null.
+        var s1 = (o as C)?.Prop;
+    }
+}

csharp/ql/test/library-tests/dataflow/nullness/maybeNullExpr.expected

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+| MaybeNullExpr.cs:10:31:10:53 | ... ? ... : ... |
+| MaybeNullExpr.cs:10:50:10:53 | null |
+| MaybeNullExpr.cs:13:30:13:38 | ... ?? ... |
+| MaybeNullExpr.cs:13:35:13:38 | null |
+| MaybeNullExpr.cs:16:17:16:22 | ... as ... |
+| MaybeNullExpr.cs:19:19:19:24 | ... as ... |

csharp/ql/test/library-tests/dataflow/nullness/maybeNullExpr.ql

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+import csharp
+import semmle.code.csharp.dataflow.Nullness
+
+from MaybeNullExpr e
+where e.fromSource()
+select e

csharp/ql/test/library-tests/dataflow/nullness/options

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+semmle-extractor-options: /nostdlib /noconfig
+semmle-extractor-options: --load-sources-from-project:${testdir}/../../../resources/stubs/_frameworks/Microsoft.NETCore.App/Microsoft.NETCore.App.csproj