[NI] Narrow nullability constraint check in incorporator

This commit is a hotfix rather then proper solution.
The source of the issue is that currently type variable fixation result
may change due to fixation order alteration for variables with the same priority.
For instance, having variables V1, V2, and proper types Type1, Type2, such that:
V1 <: Type1
V1 <: V2
Type2 <: V2
both variables will be fixed either to Type1, if V1 will be fixed first,
or to Type2 otherwise.

Since this limitation cannot be easily overcome, the taken approach is to remove
incedental constraint added after 2d5a0546 by restricting nullability constraint check
to `Nothing?` constraints only, thus postponing problematic variable fixation.
To clearify, additional constraint is correct and should cause no harm (in ideal world),
but currently its presence changes fixation order.
So without the restriction the previously used constraint from fixed outer variable
is no longer available by the time problematic variable type is being selected.

^KT-37043 Fixed

compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirOldFrontendDiagnosticsTestGenerated.java

@@ -11345,6 +11345,16 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirOldFronte
                 runTest("compiler/testData/diagnostics/tests/inference/regressions/kt36342_2.kt");
             }
 
+            @TestMetadata("kt37043.kt")
+            public void testKt37043() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/regressions/kt37043.kt");
+            }
+
+            @TestMetadata("kt37043_2.kt")
+            public void testKt37043_2() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/regressions/kt37043_2.kt");
+            }
+
             @TestMetadata("kt4420.kt")
             public void testKt4420() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/inference/regressions/kt4420.kt");