fix: Prevent exif nullptr (DEV-4521)

[siers]

Lessons learned: if a shared_ptr isn't null, that doesn't mean shared_ptr.get() isn't.

[linear]

DEV-4521 Sipi crashes when converting a PNG file with --topleft

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 23.2%. Comparing base (de9e393) to head (9b5d6dd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/SipiImage.cpp	0.0%	1 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/SipiImage.cpp	24.5% <0.0%> (-0.1%)	⬇️

... and 6 files with indirect coverage changes