Include more null checks

as per comments on the pull request https://github.com/mattwarren/RoslynClrHeapAllocationAnalyzer/commit/0f9f62fda6fee5c8ccbde5b644fb1c86d599f24d#commitcomment-8233597 and https://github.com/mattwarren/RoslynClrHeapAllocationAnalyzer/commit/0f9f62fda6fee5c8ccbde5b644fb1c86d599f24d#commitcomment-8233588