[IAST] Fix system test weak_cipher system test (#6034)

## Summary of changes
Return first frame of the stack trace as location if no suitable one was
found, but no exclusion either

## Reason for change
System tests for weak cipher were broken due to a rare situation where
no location was detected for the vulnerability

## Implementation details
If no excluded frame was found, but the rest were skipped, we are
returning the first stack frame

## Test coverage
system tests should pass now

tracer/src/Datadog.Trace/Iast/IastModule.cs

@@ -694,8 +694,7 @@ private static IastModuleResponse GetScope(string evidenceValue, IntegrationId i
     private static Location? GetLocation(StackTrace? stack = null, Span? currentSpan = null)
     {
         stack ??= StackWalker.GetStackTrace();
-        var stackFrame = StackWalker.GetFrame(stack);
-        if (stackFrame is null)
+        if (!StackWalker.TryGetFrame(stack, out var stackFrame))
         {
             return null;
         }

tracer/src/Datadog.Trace/Iast/StackWalker.cs

@@ -52,8 +52,9 @@ public static StackTrace GetStackTrace()
         return new StackTrace(DefaultSkipFrames, true);
     }
 
-    public static StackFrame? GetFrame(StackTrace stackTrace)
+    public static bool TryGetFrame(StackTrace stackTrace, out StackFrame? targetFrame)
     {
+        targetFrame = null;
         foreach (var frame in stackTrace.GetFrames())
         {
             var declaringType = frame?.GetMethod()?.DeclaringType;
@@ -62,18 +63,19 @@ public static StackTrace GetStackTrace()
             {
                 if (excludeType == declaringType?.FullName)
                 {
-                    return null;
+                    return false;
                 }
             }
 
             var assembly = declaringType?.Assembly.GetName().Name;
             if (assembly != null && !MustSkipAssembly(assembly))
             {
-                return frame;
+                targetFrame = frame;
+                break;
             }
         }
 
-        return null;
+        return true;
     }
 
     public static bool MustSkipAssembly(string assembly)