Fix potential NPE for transportType during write

[joeygrover]

Fixes #945

This PR is ready for review.

Risk

This PR makes no API changes.

Testing Plan

• Send a packet to router service to send while no transport is connected.

Summary

• If no transport type is supplied and no transport is currently connected. The write task runnable will simply return instead of trying to write the packet while not supplying a transport type.

Changelog

Bug Fixes

• There is likely a race condition where the packet write runnable can be mid or starting execution as the router service is disconnecting or already disconnected.

CLA

• I have signed the CLA

[codecov-io]

Codecov Report

Merging #956 into develop will increase coverage by 0.03%.
The diff coverage is 0%.

@@              Coverage Diff              @@
##             develop     #956      +/-   ##
=============================================
+ Coverage      42.55%   42.58%   +0.03%     
  Complexity      3735     3735              
=============================================
  Files            434      434              
  Lines          23098    23100       +2     
  Branches        2698     2698              
=============================================
+ Hits            9829     9837       +8     
+ Misses         12623    12615       -8     
- Partials         646      648       +2

Impacted Files	Coverage Δ	Complexity Δ
...om/smartdevicelink/transport/SdlRouterService.java	7.19% <0%> (-0.01%)	9 <0> (ø)
...smartdevicelink/encoder/VirtualDisplayEncoder.java	29.31% <0%> (+4.59%)	8% <0%> (ø)	⬇️

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