fix(completion): handle empty suffix and inline completion items

[lkk214]

Description

Fixed issues with using accept:

• If the current line has only one suffix, an error will occur when using accept
• The completion chars may be filtered by dropMinimum in postprocess If there are only a few completion chars at the end
  (Empty items skip the rendering process to be consistent with vscode)

Screenshots

Test

[Copilot]

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