[FIR] FirNullSymbolNamesProvider: Fix mayHaveSyntheticFunctionType

- The null symbol names provider knows nothing about the names provided
  by its symbol provider, so it also has to assume that the symbol
  provider can provide a synthetic function type.
- This inconsistency has not caused an issue so far because
  `mayHaveTopLevelClassifier` is also overridden, and
  `mayHaveSyntheticFunctionType` is not used outside that context.

compiler/fir/providers/src/org/jetbrains/kotlin/fir/resolve/providers/FirSymbolNamesProvider.kt

@@ -146,6 +146,9 @@ object FirNullSymbolNamesProvider : FirSymbolNamesProvider() {
     override val hasSpecificCallablePackageNamesComputation: Boolean get() = false
     override fun getTopLevelCallableNamesInPackage(packageFqName: FqName): Set<Name>? = null
 
+    override val mayHaveSyntheticFunctionTypes: Boolean get() = true
+    override fun mayHaveSyntheticFunctionType(classId: ClassId): Boolean = true
+
     override fun mayHaveTopLevelClassifier(classId: ClassId): Boolean = true
     override fun mayHaveTopLevelCallable(packageFqName: FqName, name: Name): Boolean = true
 }