Optimize memory footprint for SlicedMapImpl in case it's empty

It might be useful in case of many delegating traces that
remain effectively empty themselves (up to 7M of empty maps in case of compilation of 'idea' module)

compiler/frontend/src/org/jetbrains/kotlin/util/slicedMap/SlicedMapImpl.java

@@ -23,6 +23,7 @@ import com.intellij.openapi.util.Key;
 import com.intellij.util.keyFMap.KeyFMap;
 import kotlin.jvm.functions.Function3;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.Collection;
 import java.util.Collections;
@@ -31,7 +32,8 @@ import java.util.Map;
 public class SlicedMapImpl implements MutableSlicedMap {
 
     private final boolean alwaysAllowRewrite;
-    private final Map<Object, KeyFMap> map = new OpenAddressLinearProbingHashTable<>();
+    @Nullable
+    private Map<Object, KeyFMap> map = null;
     private Multimap<WritableSlice<?, ?>, Object> collectiveSliceKeys = null;
 
     public SlicedMapImpl(boolean alwaysAllowRewrite) {
@@ -44,6 +46,10 @@ public class SlicedMapImpl implements MutableSlicedMap {
             return;
         }
 
+        if (map == null) {
+            map = new OpenAddressLinearProbingHashTable<>();
+        }
+
         KeyFMap holder = map.get(key);
         if (holder == null) {
             holder = KeyFMap.EMPTY_MAP;
@@ -76,13 +82,13 @@ public class SlicedMapImpl implements MutableSlicedMap {
 
     @Override
     public void clear() {
-        map.clear();
+        map = null;
         collectiveSliceKeys = null;
     }
 
     @Override
     public <K, V> V get(ReadOnlySlice<K, V> slice, K key) {
-        KeyFMap holder = map.get(key);
+        KeyFMap holder = map != null ? map.get(key) : null;
 
         V value = holder == null ? null : holder.get(slice.getKey());
 
@@ -100,6 +106,7 @@ public class SlicedMapImpl implements MutableSlicedMap {
 
     @Override
     public void forEach(@NotNull Function3<WritableSlice, Object, Object, Void> f) {
+        if (map == null) return;
         map.forEach((key, holder) -> {
             if (holder == null) return;
 
@@ -114,6 +121,8 @@ public class SlicedMapImpl implements MutableSlicedMap {
     @NotNull
     @Override
     public <K, V> ImmutableMap<K, V> getSliceContents(@NotNull ReadOnlySlice<K, V> slice) {
+        if (map == null) return ImmutableMap.of();
+
         ImmutableMap.Builder<K, V> builder = ImmutableMap.builder();
 
         map.forEach((key, holder) -> {