Fix the issue where detection is not possible only when freq1 is used.

[cdevelop]

When only freq1 is configured and freq2 is set to zero, detection cannot be performed.
for example

<descriptors debug-level="0">
    <descriptor name="cn">
        <tone name="BUSY_TONE">
            <element freq1="450" min="300" max="400" freq2="0"/>
            <element freq1="0" min="300" max="400" freq2="0"/>
            <element freq1="450" min="300" max="400" freq2="0"/>
            <element freq1="0" min="300" max="400" freq2="0"/>
            <element freq1="450" min="300" max="400" freq2="0"/>
        </tone>
    </descriptor>
</descriptors>