Fix graph rendering issue for tensors with zero dimension size

[groszewn]

A zero dimension tensor results in a proto with an empty size field, as seen below:

node {
  name: "input/b"
  op: "IO Node"
  attr {
    key: "attr"
    value {
      s: ""
    }
  }
  attr {
    key: "_output_shapes"
    value {
      list {
        shape {
          dim {
            size: 1
          }
          dim { <<<
          }
        }
      }
    }
  }
}

This results in dim.size returning null for these particular cases, breaking graph rendering. We update to default the null case to 0.

Fixes #6418

[raphCode]

Seems I was about right with my assumption about empty protobuf messages?
I actually thought protobuf parsing code is always autogenerated and correctly populates omitted values with their defaults.

[groszewn]

@raphCode

Seems I was about right with my assumption about empty protobuf messages? I actually thought protobuf parsing code is always autogenerated and correctly populates omitted values with their defaults.

The graph plugin README specifies that we use a custom parser for this plugin. I'm not quite sure why that is the case, but that would explain the root of the issue.