Handle null metaspace values explicitly when rendering memory charts

[nathankooij]

After the upgrade to SBA 2.5.0, I noticed an error started to appear in the console when accessing the details page of an instance (notice the NaN), although rendering seems fine:

Error: <path> attribute d: Expected number, "M0,NaNL24.095068694…".

I tracked this down to the upgrade of the d3-scale library (as part of 4b72b42), where in d3/d3-scale#241 they started to treat null values as undefined which get coerced to NaN. Just like

spring-boot-admin/spring-boot-admin-server-ui/src/main/frontend/views/instances/details/mem-chart.vue

Line 81 in 9257bf2

.y0(d => y(d.metaspace || 0))

, we should also handle null values explicitly here. This also resolves the errors in my local setup.

[codecov-commenter]

Codecov Report

Merging #1801 (7a0940f) into master (9257bf2) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1801   +/-   ##
=========================================
  Coverage     83.71%   83.71%           
  Complexity     1244     1244           
=========================================
  Files           156      156           
  Lines          3628     3628           
  Branches        256      256           
=========================================
  Hits           3037     3037           
  Misses          462      462           
  Partials        129      129           

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