C#: Add ImplicitToString example in nullness tests.

Author: michaelnebel

csharp/ql/test/query-tests/Nullness/Implications.expected

@@ -1451,6 +1451,8 @@
 | GuardedString.cs:35:31:35:31 | access to local variable s | null | GuardedString.cs:7:20:7:32 | ... ? ... : ... | null |
 | GuardedString.cs:37:31:37:31 | access to local variable s | non-null | GuardedString.cs:7:20:7:32 | ... ? ... : ... | non-null |
 | GuardedString.cs:37:31:37:31 | access to local variable s | null | GuardedString.cs:7:20:7:32 | ... ? ... : ... | null |
+| ImplicitToString.cs:8:23:8:23 | access to local variable o | non-null | ImplicitToString.cs:7:20:7:23 | null | non-null |
+| ImplicitToString.cs:8:23:8:23 | access to local variable o | null | ImplicitToString.cs:7:20:7:23 | null | null |
 | NullAlwaysBad.cs:9:17:9:25 | ... != ... | false | NullAlwaysBad.cs:9:17:9:17 | access to parameter s | null |
 | NullAlwaysBad.cs:9:17:9:25 | ... != ... | true | NullAlwaysBad.cs:9:17:9:17 | access to parameter s | non-null |
 | NullAlwaysBad.cs:9:17:9:41 | ... \|\| ... | false | NullAlwaysBad.cs:9:17:9:25 | ... != ... | false |

csharp/ql/test/query-tests/Nullness/ImplicitToString.cs

@@ -0,0 +1,10 @@
+using System;
+
+class ImplicitToStringTest
+{
+    void InterpolatedStringImplicitToString()
+    {
+        object o = null;
+        string s = $"{o}"; // GOOD
+    }
+}

csharp/ql/test/query-tests/Nullness/NullAlways.expected

@@ -40,4 +40,5 @@
 | E.cs:439:13:439:13 | access to parameter s | Variable $@ is always null at this dereference. | E.cs:435:29:435:29 | s | s |
 | Forwarding.cs:36:31:36:31 | access to local variable s | Variable $@ is always null at this dereference. | Forwarding.cs:7:16:7:16 | s | s |
 | Forwarding.cs:40:27:40:27 | access to local variable s | Variable $@ is always null at this dereference. | Forwarding.cs:7:16:7:16 | s | s |
+| ImplicitToString.cs:8:23:8:23 | access to local variable o | Variable $@ is always null at this dereference. | ImplicitToString.cs:7:16:7:16 | o | o |
 | NullAlwaysBad.cs:9:30:9:30 | access to parameter s | Variable $@ is always null at this dereference. | NullAlwaysBad.cs:7:29:7:29 | s | s |

csharp/ql/test/query-tests/Nullness/NullMaybe.expected

@@ -443,6 +443,8 @@ nodes
 | GuardedString.cs:34:9:37:40 | if (...) ... |
 | GuardedString.cs:34:26:34:26 | 0 |
 | GuardedString.cs:35:31:35:31 | access to local variable s |
+| ImplicitToString.cs:7:16:7:23 | SSA def(o) |
+| ImplicitToString.cs:8:23:8:23 | access to local variable o |
 | NullAlwaysBad.cs:7:29:7:29 | SSA param(s) |
 | NullAlwaysBad.cs:9:30:9:30 | access to parameter s |
 | NullMaybeBad.cs:7:27:7:27 | access to parameter o |
@@ -837,6 +839,7 @@ edges
 | GuardedString.cs:31:26:31:27 | 10 | GuardedString.cs:34:9:37:40 | if (...) ... |
 | GuardedString.cs:34:9:37:40 | if (...) ... | GuardedString.cs:34:26:34:26 | 0 |
 | GuardedString.cs:34:26:34:26 | 0 | GuardedString.cs:35:31:35:31 | access to local variable s |
+| ImplicitToString.cs:7:16:7:23 | SSA def(o) | ImplicitToString.cs:8:23:8:23 | access to local variable o |
 | NullAlwaysBad.cs:7:29:7:29 | SSA param(s) | NullAlwaysBad.cs:9:30:9:30 | access to parameter s |
 | NullMaybeBad.cs:13:17:13:20 | null | NullMaybeBad.cs:7:27:7:27 | access to parameter o |
 | Params.cs:20:12:20:15 | null | Params.cs:14:17:14:20 | access to parameter args |