thin: guard keyword-value pair decode against empty payload bytes

[SAY-5]

Fixes #587.

_process_keyword_value_pairs left text_value and binary_value as None whenever the server sent a zero-length payload, then unconditionally called text_value.decode() for TNS_KEYWORD_NUM_CURRENT_SCHEMA and TNS_KEYWORD_NUM_EDITION. ALTER PLUGGABLE DATABASE OPEN/CLOSE against 19c reproduces the path — the DDL succeeded server-side but the async client crashed in the response parser and left the connection unusable.

Add the obvious nil-guards on each branch (also on the TNS_KEYWORD_NUM_TRANSACTION_ID binary path).

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[SAY-5]

The failing oca/oracle check is the OCA contributor signature step; I will get that signed and rerun.