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/cfg/initBlock.dot

@@ -84,7 +84,7 @@ digraph initBlock_kt {
     20 -> {21};
     21 -> {22};
     22 -> {23};
-    23 -> {28};
+    23 -> {28}    [label=onUncaughtException];
     23 -> {24} [style=dotted];
     24 -> {25} [style=dotted];
     25 -> {26} [style=dotted];