[FIR] Make UNSAFE_INFIX_CALL and UNSAFE_OPERATOR_CALL parameter nullable

This fixes an NPE in the IDE in presence of a syntax error when the
argument expression has no source.

#KT-64062 Fixed

compiler/testData/diagnostics/tests/nullableTypes/unsafeCallOnOperatorRedCode.fir.kt

@@ -0,0 +1,3 @@
+fun add(a: Int?, b: Int?): Int {
+    return a<!UNSAFE_OPERATOR_CALL!>+<!><!SYNTAX!><!>
+}

compiler/testData/diagnostics/tests/nullableTypes/unsafeCallOnOperatorRedCode.kt

@@ -0,0 +1,3 @@
+fun add(a: Int?, b: Int?): Int {
+    return a<!NONE_APPLICABLE!>+<!><!SYNTAX!><!>
+}