Don't try to remove negative runoff on an absent field

I'm not sure if this is necessary, but it seems like it could be in some
configurations, and it won't hurt to do this check.

mediator/med_phases_post_rof_mod.F90

@@ -120,6 +120,7 @@ subroutine med_phases_post_rof(gcomp, rc)
     real(r8), pointer   :: data_orig(:)
     real(r8), pointer   :: data_copy(:)
     integer             :: n
+    logical             :: exists
     character(len=*), parameter :: subname='(med_phases_post_rof)'
     !---------------------------------------
 
@@ -144,8 +145,12 @@ subroutine med_phases_post_rof(gcomp, rc)
 
     if (remove_negative_runoff) then
       do n = 1, size(fields_to_remove_negative_runoff)
-        call med_phases_post_rof_remove_negative_runoff(gcomp, fields_to_remove_negative_runoff(n), rc)
+        call ESMF_FieldBundleGet(FBrof_r, fieldName=trim(fields_to_remove_negative_runoff(n)), isPresent=exists, rc=rc)
         if (ChkErr(rc,__LINE__,u_FILE_u)) return
+        if (exists) then
+          call med_phases_post_rof_remove_negative_runoff(gcomp, fields_to_remove_negative_runoff(n), rc)
+          if (ChkErr(rc,__LINE__,u_FILE_u)) return
+        end if
       end do
     end if