[FIR] Make source in qualifier parts nullable

This is needed to be able to create no-source user type refs from plugins

compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirQualifierPart.kt

@@ -9,12 +9,12 @@ import org.jetbrains.kotlin.KtSourceElement
 import org.jetbrains.kotlin.name.Name
 
 interface FirTypeArgumentList {
-    val source: KtSourceElement
+    val source: KtSourceElement?
     val typeArguments: List<FirTypeProjection>
 }
 
 interface FirQualifierPart {
-    val source: KtSourceElement
+    val source: KtSourceElement?
     val name: Name
     val typeArgumentList: FirTypeArgumentList
 }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirQualifierPartImpl.kt

@@ -11,12 +11,12 @@ import org.jetbrains.kotlin.fir.types.FirTypeArgumentList
 import org.jetbrains.kotlin.fir.types.FirTypeProjection
 import org.jetbrains.kotlin.name.Name
 
-class FirTypeArgumentListImpl(override val source: KtSourceElement) : FirTypeArgumentList {
+class FirTypeArgumentListImpl(override val source: KtSourceElement?) : FirTypeArgumentList {
     override val typeArguments = mutableListOf<FirTypeProjection>()
 }
 
 class FirQualifierPartImpl(
-    override val source: KtSourceElement,
+    override val source: KtSourceElement?,
     override val name: Name,
     override val typeArgumentList: FirTypeArgumentList
 ) : FirQualifierPart