wallet: fix only default RBF to true when unset #35677

In SetFeeEstimateMode(), m_signal_bip125_rbf should only be set to the default value (true) when it is unset.

Currently, the code uses !m_signal_bip125_rbf, which evaluates to true both when the std::optional is unset and when it contains false. As a result, an explicitly set false is incorrectly overwritten with true.

Fix this by checking has_value() instead, so only the unset case receives the default value.

For extra context see the PR where it was detected #35433 (review)

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35677.

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???

In C++, the behavior of operator! on a std::optional does not evaluate the contained value. Instead, according to the C++ standard, !m_signal_bip125_rbf is equivalent to checking if the optional does not contain a value (!m_signal_bip125_rbf.has_value()).

Generally, it would be good if refactor pull requests said that they are refactor, and behavior changing pull requests gave steps to test.

I think I just got confused for a second 😅