False Negative: NULL_DEREFERENCE missing when a field is explicitly assigned null in the constructor

**Description**
Infer fails to report a NULL_DEREFERENCE (or NULLPTR_DEREFERENCE) when a class field is explicitly initialized to null within the constructor and subsequently dereferenced in another instance method.

**To Reproduce**
``` Java
class Test {
String x;

// Case 1
public Test() {
this.x = null;
}
public void testExplicitField() {
x.trim(); //<-should report (FN)
}
// Case 2
public void testLocalDereference() {
String y = null;
y.length(); // <-reported (TP)
}
}
```
**Expected behavior**
Infer should report a NULL_DEREFERENCE for the line x.trim(); in testExplicitField(). The analysis should be able to propagate the null state from the constructor to the method call for the same instance.

**Actual behavior**
Infer only reports the issue in Case2. The dereference of the field x in Case1 is silently ignored, even though it was explicitly assigned null in the class's own constructor.