Add comment to ignored default case

compiler/ballerina-lang/src/main/java/org/wso2/ballerinalang/compiler/semantics/analyzer/TypeChecker.java

@@ -1573,6 +1573,7 @@ private boolean isConstExpression(BLangExpression expression) {
                             }
                         }
                         default -> {
+                            // Ignore the default case
                         }
                     }
                 }