Improvements for null assertions.

- isset($a->b->c) now asserts that $a is not null, $a->b is not null, and $a->b->c is not null
- Consistent with php, !is_null($a->b->c) asserts the same, but does report null dereferences in the parameter before it asserts.
- To avoid errors use !is_null($a?->b?->c) or isset($a->b->c).
- Don't assert when "$this instanceof" is used.  Ie. $this instanceOf Foo from inside of Bar does not change $this to be a Foo.

tests/units/Checks/TestData/TestComplexTypes.5.inc

@@ -0,0 +1,13 @@
+<?php
+
+class TestClass {
+
+	private string|array $testArray = "foo";
+
+	function method(): array {
+		if (!is_array($this->testArray)) {
+			$this->testArray = explode(',', $this->testArray);
+		}
+		return $this->testArray;
+	}
+}

tests/units/Checks/TestData/TestComplexTypes.6.inc

@@ -0,0 +1,14 @@
+<?php
+
+class TestClass {
+
+	private $testArray;
+
+	function method(): array {
+		$this->testArray->foo = true ? 'bar' : [];
+		if (!is_array($this->testArray->foo)) {
+			$this->testArray->foo = explode(',', $this->testArray->foo);
+		}
+		return $this->testArray;
+	}
+}

tests/units/Checks/TestData/TestNullablePropertiesCheck.4.inc

@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+function nullError(?int $a):int {
+	if (is_null($a)) {
+		return 0;
+	}
+	return $a;
+}
+
+nullError(null);

tests/units/Checks/TestData/TestNullablePropertiesCheck.5.inc

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+class NullDefault {
+	public ?int $a;
+
+	function init():int {
+		if (is_null($this->a)) {
+			$this->a = 0;
+		}
+		return $this->a;
+	}
+}