[JDBC] Follow-ups to the hostnameOverride connection property #1442

Follow-up to #1363, which added the hostnameOverride connection property (#1319).

• The ArrowFlightSqlClientHandler.Builder copy constructor copies hostnameOverride, although the override is only valid for the original host and port. getStreams, the only caller, resets it today, but a future caller that copies the builder and changes the host or port would verify that endpoint against the override.
• ArrowFlightConnectionConfigImpl.getHostnameOverride() returns "" for hostnameOverride=, while its Javadoc says it returns null. Only the builder normalizes blank values.
• testHostnameOverrideIsIgnoredWithoutEncryption passes whatever happens to the override: it makes no RPC, and a plaintext channel accepts any authority.
• The docs don't say that the override replaces the whole :authority, so the port is no longer sent.