fix: skip RPC tests when TESTNET/DEVNET URL is set but empty

[radhika-kapoor]

Problem

The test_matrices_testnet and test_matrices_devnet tests were failing in CI with:

Error: relative URL without a base
Location: crates/node/tests/it/tempo_transaction/rpc.rs:83:19

When a GitHub Actions secret (e.g. TEMPO_TESTNET_RPC_URL) is unavailable for a PR,
GitHub expands it to an empty string "" rather than leaving the variable unset.
std::env::var returns Ok("") for a set-but-empty variable , not Err(_) , so the
old guard fell through to Self::connect(""), which panicked immediately trying to
parse "" as a URL.

Fix

Added .and_then(|u| if u.is_empty() { Err(std::env::VarError::NotPresent) } else { Ok(u) })
before the match in both testnet() and devnet(). This converts Ok("") to
Err(NotPresent) before the match, so the Err(_) arm handles it the same way as a
truly unset variable — returning Ok(None) and skipping the test gracefully.

env var state	result
not set	skip
set to ""	skip (fixed)
set to a real URL	connect & run

Test plan

• Verified failure locally with TEMPO_TESTNET_RPC_URL="" TEMPO_DEVNET_RPC_URL="" cargo nextest run --profile ci-rpc -E 'package(tempo-node) & binary(it) & test(/test_matrices_testnet|test_matrices_devnet/)'
  -> 4 retries, all fail with relative URL without a base
• Verified fix locally with same command -> test passes with graceful skip