Correct some issues with `g2p generate-mapping`

[dhdaines]

Fixes #353 (do not try to generate mappings for empty symbols) Fixes #354 (do not copy the rules file name when generating mapping)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.28%. Comparing base (43d3d37) to head (ea04262).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #356 +/- ## ========================================== - Coverage 93.40% 93.28% -0.12% ========================================== Files 18 18 Lines 2336 2339 +3 Branches 517 519 +2 ========================================== Hits 2182 2182 - Misses 88 90 +2 - Partials 66 67 +1 ```

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[github-actions[bot]]

CLI load time: 0:00.05
PR head ea042628b9419eb3844b67b63d68cc8875723542
Imports that take more than 0.1 s:
import time: self [us] | cumulative | imported package