Empty array handling for ReportTemplate.html

[rberghuis]

The Mobile Devices diagram received data but did no validation before handing it to the library. Two things were missing:

1. It passed zero-value links
If a tenant has no mobile devices, all the mobile links have value 0. Those zeros were passed straight to d3-sankey, which tried to draw a chart with no meaningful data.

2. It had no guard for empty data.
When all links are zero, filtering them out leaves an empty dataset. d3-sankey then tried to compute new Array(max_depth + 1) where max_depth is undefined (no nodes to measure), which evaluates to new Array(NaN), an invalid operation that crashes the whole page.

The fix adds two lines of defence:

1. Strip out any links with a value of zero or null before doing anything else.
2. If nothing is left after stripping, show "No data available." instead of attempting to render the chart.

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