Support for NULL records in log/graph when timeouts occur

[Daniel-Beardsmore]

MRTG only permits timeouts to be logged as 0 or the previous value. Such entries should be treated as NULL (undef), so that there is no need to choose between two different ways of deliberately recording wrong data. The graph then ought to use something like an orange background for each column of pixels where there is no data at all, so that there is no confusion as to whether there is or is not any valid data.

Within the log, a value of “-” should be a simple-enough way to indicate that no data was returned at all.

[Daniel-Beardsmore]

(Note, this is for rateup)