OUSD rebases can fail if there are no rebasingTokens

In the unlikely event that all OUSD is nonRebasing, then rebases revert, blocking redeems and large mints.

This isn't something that's going to happen in real life, but it's an incorrectness that would be nice to fix someday as a part of renaming the rebasing part of OUSD.

  it("Rebasing should not fail when all OUSD is non-rebasing.", async () => {
    let { ousd, vault, matt, josh, usdc, mockNonRebasing } = await loadFixture(
      defaultFixture
    );
    await ousd.connect(matt).transfer(mockNonRebasing.address, ousdUnits("100"));
    await ousd.connect(josh).transfer(mockNonRebasing.address, ousdUnits("100"));
    await usdc.mintTo(vault.address, usdcUnits("10"));
    await vault.rebase(); // Revert
   // Should succeed and keep rebasing tokens per credit the same.
  })

My initial lean is towards just short-circuit out of rebasing if rebasingTokens = 0, similar to what we do if the token amount has not changed.

Search the contracts and tests for rebasingTokens and the vault.rebase() entry point, then reproduce the supplied fixture with all OUSD transferred to mockNonRebasing. Add or update the regression test so the rebase succeeds and preserves the rebasing-token credit when no rebasing tokens remain.