Properly handle empty dictionary in get_range_spectrogram

[iaraota]

This PR addresses the bug introduced in #403 by acknowledging that globalv.SPECTROGRAMS might not contain the specified key. In such cases, it now correctly returns an empty SpectrogramList(), according to the previously removed code snippet:

return globalv.SPECTROGRAMS.get(key, SpectrogramList())

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 49.54%. Comparing base (720bd0c) to head (6233e7a). Report is 4 commits behind head on master.

Files	Patch %	Lines
gwsumm/data/range.py	0.00%	12 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #407 +/- ## ========================================== - Coverage 49.64% 49.54% -0.10% ========================================== Files 60 60 Lines 8823 8831 +8 ========================================== - Hits 4380 4375 -5 - Misses 4443 4456 +13 ``` | [Flag](https://app.codecov.io/gh/gwpy/gwsumm/pull/407/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gwpy) | Coverage Δ | | |---|---|---| | [Linux](https://app.codecov.io/gh/gwpy/gwsumm/pull/407/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gwpy) | `49.54% <0.00%> (-0.10%)` | :arrow_down: | | [macOS](https://app.codecov.io/gh/gwpy/gwsumm/pull/407/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gwpy) | `49.54% <0.00%> (?)` | | | [python3.10](https://app.codecov.io/gh/gwpy/gwsumm/pull/407/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gwpy) | `49.54% <0.00%> (-0.10%)` | :arrow_down: | | [python3.11](https://app.codecov.io/gh/gwpy/gwsumm/pull/407/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gwpy) | `49.54% <0.00%> (-0.10%)` | :arrow_down: | | [python3.9](https://app.codecov.io/gh/gwpy/gwsumm/pull/407/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gwpy) | `49.54% <0.00%> (-0.10%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gwpy#carryforward-flags-in-the-pull-request-comment) to find out more.

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