[LL] Avoid exceptions when analyzing inconsistent PSI in LLFirDeclarationModificationService

- The exceptions in KT-63130 occur because PSI tree change events may be
  published when parts of the PSI under modification are inconsistent.
  Such inconsistent elements are then analyzed by
  `LLFirDeclarationModificationService` to check if the modification
  occurs in a contract statement.
- The solution adds `*OrNull` functions to `KtQualifiedExpression` which
  return `null` instead of failing with an exception when the receiver
  or selector cannot be found.

^KT-63130 fixed

analysis/low-level-api-fir/testData/declarationModificationService/psiResilience/dotQualifiedExpression/contractCall.kt

@@ -1,5 +1,3 @@
-// IGNORE_FIR
-
 import kotlin.contracts.InvocationKind
 
 inline fun foo(block: () -> Unit) {

analysis/low-level-api-fir/testData/declarationModificationService/psiResilience/dotQualifiedExpression/stringLengthFirstStatement.kt

@@ -1,5 +1,3 @@
-// IGNORE_FIR
-
 fun foo(string: String) {
     <expr>string.length</expr>
 }