Fix OracleToOracleOperator source_sql_params default before rendering

[baha-bouali]

Fixes the oracle provider's entry from the #70296 exemption-list burn-down: OracleToOracleOperator applied a default-value coercion to a template field inside __init__, before Jinja templating runs.

• OracleToOracleOperator: source_sql_params was defaulted from None to {} in __init__, against the raw un-rendered value. Moved the default to execute().

__init__ now only does a plain assignment; the field-dependent logic runs in execute(), after templating.
Test added covering the fixed behavior.
Related to #70296

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[boring-cyborg]

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[shahar1]

Could you please remove the comments?