fix: Add fallback for currentBufferTarget

[metal-messiah]

Add a fallback to the currentBufferTarget for cases where the preload buffer has been cleared

Overview

This PR sets the currentBufferTarget to the preload buffer only if it exists. This should alleviate an edge-case issue where the payloadBytesEstimation throws an error because the buffer target does not exist.

Related Issue(s)

https://new-relic.atlassian.net/browse/NR-215642

[github-actions]

Asset Size Report

Merging this pull request will result in the following asset size changes:

Agent	Asset	Previous Size	New Size	Diff
lite	loader	30.51 kB / 10.65 kB (gzip)	30.51 kB / 10.65 kB (gzip)	0% / 0% (gzip)
lite	async-chunk	45.85 kB / 15.1 kB (gzip)	45.85 kB / 15.1 kB (gzip)	0% / 0% (gzip)
pro	loader	49.15 kB / 16.73 kB (gzip)	49.15 kB / 16.73 kB (gzip)	0% / 0% (gzip)
pro	async-chunk	73.11 kB / 23.11 kB (gzip)	73.11 kB / 23.11 kB (gzip)	0% / 0% (gzip)
spa	loader	55.63 kB / 18.71 kB (gzip)	55.63 kB / 18.7 kB (gzip)	0% / -0.01% (gzip)
spa	async-chunk	87.87 kB / 27.51 kB (gzip)	87.87 kB / 27.51 kB (gzip)	0% / 0% (gzip)
lite-polyfills	loader	125.13 kB / 39.86 kB (gzip)	125.13 kB / 39.86 kB (gzip)	0% / 0% (gzip)
lite-polyfills	async-chunk	58.21 kB / 17.2 kB (gzip)	58.21 kB / 17.2 kB (gzip)	0% / 0% (gzip)
pro-polyfills	loader	144.69 kB / 45.62 kB (gzip)	144.69 kB / 45.62 kB (gzip)	0% / 0% (gzip)
pro-polyfills	async-chunk	101.78 kB / 27.58 kB (gzip)	101.78 kB / 27.58 kB (gzip)	0% / 0% (gzip)
spa-polyfills	loader	152.71 kB / 47.76 kB (gzip)	152.71 kB / 47.76 kB (gzip)	0% / 0% (gzip)
spa-polyfills	async-chunk	117.34 kB / 32.56 kB (gzip)	117.34 kB / 32.56 kB (gzip)	0% / 0% (gzip)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8e5a877) 78.66% compared to head (416d949) 78.66%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #861   +/-   ##
=======================================
  Coverage   78.66%   78.66%           
=======================================
  Files         144      144           
  Lines        6404     6404           
  Branches     1251     1251           
=======================================
  Hits         5038     5038           
  Misses       1158     1158           
  Partials      208      208           

Flag	Coverage Δ
unit-tests	55.33% <100.00%> (ø)

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