[BufferMessageEncoder] Reduce the number of force unwraps

[fabianfett]

Motivation

The fewer force unwraps, the better

Changes

• Remove an Optional return type, that doesn't need to be an optional return type

Result

Safer code, thanks to fewer force unwraps.

[codecov-commenter]

Codecov Report

Merging #234 (38c980f) into main (4cd1567) will increase coverage by 4.06%.
The diff coverage is 46.15%.

@@            Coverage Diff             @@
##             main     #234      +/-   ##
==========================================
+ Coverage   39.83%   43.89%   +4.06%     
==========================================
  Files         116      116              
  Lines        9417     9420       +3     
==========================================
+ Hits         3751     4135     +384     
+ Misses       5666     5285     -381     

Flag	Coverage Δ
unittests	43.89% <46.15%> (+4.06%)	⬆️

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

Impacted Files	Coverage Δ
Sources/PostgresNIO/New/PSQLChannelHandler.swift	60.53% <41.66%> (+10.86%)	⬆️
...urces/PostgresNIO/New/BufferedMessageEncoder.swift	100.00% <100.00%> (+8.00%)	⬆️
Sources/PostgresNIO/New/PSQLRow-multi-decode.swift	4.44% <0.00%> (+0.19%)	⬆️
Sources/PostgresNIO/Data/PostgresRow.swift	50.00% <0.00%> (+1.02%)	⬆️
...s/PostgresNIO/New/Data/Array+PostgresCodable.swift	94.68% <0.00%> (+1.06%)	⬆️
...rces/PostgresNIO/New/Extensions/Logging+PSQL.swift	19.60% <0.00%> (+1.42%)	⬆️
Sources/PostgresNIO/New/PSQLConnection.swift	30.00% <0.00%> (+2.10%)	⬆️
...ostgresNIO/Deprecated/PostgresMessageDecoder.swift	95.45% <0.00%> (+2.27%)	⬆️
...es/PostgresNIO/New/PSQLBackendMessageDecoder.swift	93.28% <0.00%> (+2.98%)	⬆️
...urces/PostgresNIO/New/Messages/ErrorResponse.swift	32.30% <0.00%> (+3.07%)	⬆️
... and 22 more

[gwynne]

Thank you 😭❤️