Fix HELLO command handler for empty command input

[cmaglie]

Previously the discovery handler would not gracefully handle an empty HELLO command:

$ ./dummy-discovery
HELLO
panic: runtime error: slice bounds out of range [6:5]

goroutine 1 [running]:
github.com/arduino/pluggable-discovery-protocol-handler/v2.(*Server).Run(0xc00002c1e0, {0x5147c8?, 0xc000012010?}, {0x5147e8?, 0xc000012018?})
        /home/megabug/Workspace/pluggable-discovery-protocol-handler/discovery.go:135 +0x5a5
main.main()
        /home/megabug/Workspace/pluggable-discovery-protocol-handler/dummy-discovery/main.go:44 +0x88
$

After this patch, it is handled correctly:

$ ./dummy-discovery 
HELLO
{
  "eventType": "hello",
  "message": "Invalid HELLO command",
  "error": true
}
QUIT
{
  "eventType": "quit",
  "message": "OK"
}
$

Fix #32

[codecov-commenter]

Codecov Report

Base: 0.00% // Head: 0.00% // No change to project coverage 👍

Coverage data is based on head (71b0ce2) compared to base (69bcf9a).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #33   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files          4       4           
  Lines        255     259    +4     
=====================================
- Misses       255     259    +4     

Flag	Coverage Δ
unit	0.00% <0.00%> (ø)

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Impacted Files	Coverage Δ
discovery.go	0.00% <0.00%> (ø)

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[per1234]

Thanks Cristian!