Fixes assert in type_check_encode_buffer_empty.

[esdrubal]

Description

Assert in type_check_encode_buffer_empty was causing the compiler to crash.

We now throw a CompileError to avoid crashing.

Fixes #6338.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[github-actions]

Benchmark for 204ebcd

Click to view benchmark

Test	Base	PR	%
code_action	5.2±0.07ms	5.3±0.09ms	+1.92%
code_lens	299.7±8.57ns	297.3±5.61ns	-0.80%
compile	2.6±0.06s	2.7±0.04s	+3.85%
completion	4.7±0.03ms	4.9±0.13ms	+4.26%
did_change_with_caching	2.7±0.04s	2.7±0.07s	0.00%
document_symbol	881.5±29.54µs	860.8±25.21µs	-2.35%
format	71.4±0.75ms	72.9±0.75ms	+2.10%
goto_definition	344.2±8.26µs	361.1±12.14µs	+4.91%
highlight	9.0±0.09ms	9.1±0.30ms	+1.11%
hover	494.9±11.49µs	527.2±7.16µs	+6.53%
idents_at_position	120.6±0.55µs	118.0±0.77µs	-2.16%
inlay_hints	669.1±16.89µs	658.4±23.80µs	-1.60%
on_enter	2.2±0.17µs	2.1±0.05µs	-4.55%
parent_decl_at_position	3.7±0.01ms	3.8±0.12ms	+2.70%
prepare_rename	341.1±5.90µs	355.5±8.78µs	+4.22%
rename	9.3±0.01ms	9.3±0.13ms	0.00%
semantic_tokens	1214.9±10.07µs	1303.3±21.24µs	+7.28%
token_at_position	333.5±2.25µs	352.6±3.21µs	+5.73%
tokens_at_position	3.7±0.02ms	3.8±0.11ms	+2.70%
tokens_for_file	397.4±2.13µs	411.1±4.29µs	+3.45%
traverse	38.6±0.84ms	38.0±0.45ms	-1.55%

[github-actions]

Benchmark for 334fdd1

Click to view benchmark

Test	Base	PR	%
code_action	5.1±0.01ms	5.2±0.10ms	+1.96%
code_lens	291.8±7.35ns	285.5±8.88ns	-2.16%
compile	2.6±0.05s	2.6±0.04s	0.00%
completion	4.6±0.01ms	4.6±0.03ms	0.00%
did_change_with_caching	2.6±0.03s	2.6±0.02s	0.00%
document_symbol	915.9±19.15µs	848.7±25.55µs	-7.34%
format	72.3±0.91ms	72.1±0.64ms	-0.28%
goto_definition	338.3±10.63µs	336.1±6.38µs	-0.65%
highlight	8.9±0.14ms	8.9±0.02ms	0.00%
hover	488.6±6.20µs	488.0±4.08µs	-0.12%
idents_at_position	118.6±0.58µs	118.7±0.52µs	+0.08%
inlay_hints	632.1±23.96µs	634.5±15.66µs	+0.38%
on_enter	2.1±0.06µs	2.0±0.04µs	-4.76%
parent_decl_at_position	3.7±0.05ms	3.7±0.15ms	0.00%
prepare_rename	334.7±8.69µs	339.0±9.99µs	+1.28%
rename	9.3±0.11ms	9.3±0.17ms	0.00%
semantic_tokens	1146.4±25.97µs	1227.2±15.88µs	+7.05%
token_at_position	331.9±4.16µs	334.5±1.97µs	+0.78%
tokens_at_position	3.7±0.05ms	3.7±0.03ms	0.00%
tokens_for_file	395.5±2.55µs	397.7±4.17µs	+0.56%
traverse	37.0±0.70ms	37.6±1.06ms	+1.62%