Minor cleanup at ConstraintInjector.kt

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/components/ConstraintInjector.kt

@@ -322,9 +322,11 @@ class ConstraintInjector(
             }
 
             val constraintSets = stackForConstraintsSetsFromCurrentForkPoint?.popLast()
-            if (constraintSets == null || constraintSets.isEmpty()) return isThereSuccessfulFork
 
-            if (constraintSets.size > 1) {
+            when {
+                // Just an optimization
+                constraintSets.isNullOrEmpty() -> return isThereSuccessfulFork
+                constraintSets.size > 1 -> {
                     if (forkPointsData == null) {
                         forkPointsData = SmartList()
                     }
@@ -332,13 +334,16 @@ class ConstraintInjector(
                         constraintSets
                     )
                     return true
-            } else if (constraintSets.size == 1) {
+                }
+                else -> {
+                    // The emptiness case has been already handled above
                     processGivenForkPointBranchConstraints(
                         c,
                         constraintSets.single(),
                         position,
                     )
                 }
+            }
 
             return isThereSuccessfulFork
         }