fix: don't build proxy agent when env 'http_proxy' is empty

[lilasxie]

Running fuelup component list with empty http_proxy environment value will cause failed to get latest version info.

We should check http_proxy environment value before building proxy agent.

Installed:
  forc 0.46.1 (latest: failed to get latest version)
    - forc-fmtforc-lspforc-docforc-deployforc-run
  forc-explore 0.28.1 (latest: failed to get latest version)
  forc-index 0.23.0 (latest: failed to get latest version)
  forc-wallet 0.3.0 (latest: failed to get latest version)
  fuel-core 0.20.5 (latest: failed to get latest version)
  fuel-core-keygen : failed getting current version
  fuel-indexer 0.23.0 (latest: failed to get latest version)

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[sdankel]

Could you paste the full command you're using to repro this?

[lilasxie]

Could you paste the full command you're using to repro this?

This maybe not bug for fuelup, but for a wrong http proxy setting. As I unset http proxy with export http_proxy="" by mistake, and cause the problem. But it would be better if we check the empty http proxy setting in my opinion.