FIR CFG: correct edge label from a node that returns Nothing

If it's not within a try/catch/finally, that should be an uncaught
exception path.

^KT-45327 Fixed

compiler/fir/analysis-tests/testData/resolve/smartcasts/smartCastInInit.dot

@@ -40,7 +40,7 @@ digraph smartCastInInit_kt {
     }
     6 -> {7};
     7 -> {8};
-    8 -> {13};
+    8 -> {13}    [label=onUncaughtException];
     8 -> {9} [style=dotted];
     9 -> {10} [style=dotted];
     10 -> {13 11} [style=dotted];