Pass in null for TraceFlag StartDate

[JimSuplizio]

What does this PR do?

When creating/updating the TraceFlag's StartTime use null instead of the local system time on whatever machine happens to be running VS Code. The problem here is that that local machine and appserver are more than likely not going to be in sync. This means that creating TraceFlag StartTime from the local system time, especially one whose time is ahead of the appserver, would end up causing a delay for when logging started. Instead, passing null for the start time which will tell logging to use the now time on the server side which means the very next command will log appropriately.

What issues does this PR fix or reference?

@W-5540032@

[codecov]

Codecov Report

Merging #743 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #743   +/-   ##
========================================
  Coverage    74.23%   74.23%           
========================================
  Files          163      163           
  Lines         6609     6609           
  Branches      1039     1039           
========================================
  Hits          4906     4906           
  Misses        1433     1433           
  Partials       270      270

Impacted Files	Coverage Δ
...de-core/src/commands/forceStartApexDebugLogging.ts	46.15% <ø> (ø)	⬆️

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[lcampos]

Fix for #710

[salesforce-cla]

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