Fix the problem of determining whether there is a MetadataSeparator error

[saber-wang]

Description

I forgot a merge request when I deleted the branch yesterday😂😂

Judge logic error here and TopicAttribute.Metadataseparator default should be null

dotnet-sdk/src/Dapr.AspNetCore/DaprEndpointRouteBuilderExtensions.cs

Line 100 in 927daa4

var metadataSeparator = e.FirstOrDefault(e => string.IsNullOrEmpty(e.MetadataSeparator)).MetadataSeparator ?? ",";

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #901

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation

[codecov]

Codecov Report

Merging #900 (e26a9bf) into master (d061164) will not change coverage.
The diff coverage is 80.00%.

@@           Coverage Diff           @@
##           master     #900   +/-   ##
=======================================
  Coverage   70.11%   70.11%           
=======================================
  Files         152      152           
  Lines        4939     4939           
  Branches      543      543           
=======================================
  Hits         3463     3463           
  Misses       1349     1349           
  Partials      127      127           

Flag	Coverage Δ
net5	70.07% <80.00%> (ø)
net6	70.01% <80.00%> (ø)
netcoreapp3.1	70.05% <80.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Dapr.AspNetCore/TopicAttribute.cs	77.77% <75.00%> (ø)
...r.AspNetCore/DaprEndpointRouteBuilderExtensions.cs	92.72% <100.00%> (ø)

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[halspang]

Small comment asking for an additional test case.