Default MIDI learn values are used when a snapshot has cleared them

[riban-bw]

Describe the bug When loading a snapshot that includes controllers with MIDI learn CC overriden with empty CC, the original default CC are assigned.

To Reproduce Steps to reproduce the behaviour:

1. Add Fluidsynth to a chain
2. Enable MIDI learn for volume
3. Cancel MIDI learn to clear the volume's learned CC
4. Save as snapshot
5. Restore snapshot

Expected behaviour Volume MIDI learn is empty.

Actual behaviour Volume MIDI learn is reset to CC7.

Setup:

• HARDWARE Raspberry Pi 4 Model B Rev 1.4 Audio: ZynADAC Display: ZynScreen 3.5 (v1) Wiring: MCP23017_ZynScreen I2C: MCP23017@0x20, MCP23008@0x21, ADS1115@0x48, MCP4728@0x60
• SYSTEM Raspbian GNU/Linux 10 (buster) Build Date: 2020-09-05 Memory: 6% (126M/1872M) SD Card: 70% (19G/29G) Temperature: 61.8ºC Overclock: Medium USB Storage: 5% (9.7G/235G)
• MIDI Profile: default.sh Tuning: 440 Hz Receive Mode: Stage (Omni On) ZS3 Sub-SnapShots: On MIDI to Output: On Master Channel: 16
• SOFTWARE zyncoder: testing (6eb5496) zynthian-ui: testing (68c0485) zynthian-sys: testing (af606bf) zynthian-data: testing (45b44f9) zynthian-webconf: testing (706d78d)

[riban-bw]

Maybe perform midi learn clean before restoring snapshot learn parameters.