Remove redundant null checks for atmos (#2703)

* Remove redundant null checks for atmos

* Remove unnecessary nullability parameter from AtmosphereSystem.GetGridAtmosphere

* Remove more nullability markers

* Bring back null checks for gas tanks

* Remove null checks from GasMixture.Merge and TileAtmosphere.AssumeAir

Co-authored-by: Vera Aguilera Puerto <6766154+Zumorica@users.noreply.github.com>

Content.Server/Atmos/TileAtmosphere.cs

@@ -1121,7 +1121,7 @@ namespace Content.Server.Atmos
 
         public bool AssumeAir(GasMixture giver)
         {
-            if (giver == null || Air == null) return false;
+            if (Air == null) return false;
 
             Air.Merge(giver);