PS /Users/greg> $apitoken = "yourapitoken"
PS /Users/greg> $headers = @{ Authorization = "token $apitoken" }
PS /Users/greg> $resp = Invoke-RestMethod -Uri https://api.github.com/notifications -Headers $headers
PS /Users/greg> $resp -eq $null                                                                                                     
PS /Users/greg>

PS /Users/greg> $resp -is [System.Object]                                                                                           
True
PS /Users/greg> 

PS /Users/greg> $obj = New-Object System.Object                                                                                     
PS /Users/greg> $obj -eq $null                                                                                                      
False
PS /Users/greg> 

PS /Users/greg> $resp -eq $null
True
PS /Users/greg>

PS /Users/greg> $resp -eq $null
PS /Users/greg>

Name                           Value                                                                                               
----                           -----                                                                                               
PSVersion                      6.0.0-beta.9                                                                                        
PSEdition                      Core                                                                                                
GitCommitId                    v6.0.0-beta.9                                                                                       
OS                             Darwin 17.2.0 Darwin Kernel Version 17.2.0: Fri Sep 29 18:27:05 PDT 2017; root:xnu-4570.20.62~3/R...
Platform                       Unix                                                                                                
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}                                                                             
PSRemotingProtocolVersion      2.3                                                                                                 
SerializationVersion           1.1.0.1                                                                                             
WSManStackVersion              3.0