Consider using default value instead of hardcoded '-' in MetricColumn.GetValue()

[sleemer]

What we have currently:

 Method |       Mean |    StdDev |  Gen 0 | Allocated |
---------- |----------- |---------- |------- |---------- |
 Iterative | 31.0739 ns | 0.1091 ns |      - |       0 B |
      LINQ | 83.0435 ns | 1.0103 ns | 0.0069 |      32 B | 

What we need:

 Method |       Mean |    StdDev |  Gen 0 | Allocated |
---------- |----------- |---------- |------- |---------- |
 Iterative | 31.0739 ns | 0.1091 ns | 0.0000 |       0 B |
      LINQ | 83.0435 ns | 1.0103 ns | 0.0069 |      32 B | 

Why we need it:
We run benchmarks as a step of our TeamCity build. And having consistent csv report file will simplify exporting results into influxdb using csv plugin for telegraf.

[AndreyAkinshin]

@sleemer thanks for the request!
@adamsitnik what do you think about it?

[adamsitnik]

I am ok with changing the CSV file output to be more consistent, but I would prefer not to change the way we display the results in the console/GH/HTML. I like the fact that we display - - it's more obvious and easier to read (I don't need to look at the digits)

[AndreyAkinshin]

@sleemer is it OK if we change the default value only in the CSV reports?

[sleemer]

@AndreyAkinshin, absolutely! That’s exactly what I meant. Sorry, if it wasn’t clear from my original proposal.