clang-tidy: use nullptr

[neheb]

Found with modernize-use-nullptr

Signed-off-by: Rosen Penev [email protected]

[codecov]

Codecov Report

Merging #1678 (19fad47) into main (86c71ae) will decrease coverage by 0.00%.
The diff coverage is 59.49%.

@@            Coverage Diff             @@
##             main    #1678      +/-   ##
==========================================
- Coverage   66.90%   66.90%   -0.01%     
==========================================
  Files         151      151              
  Lines       20725    20786      +61     
==========================================
+ Hits        13867    13907      +40     
- Misses       6858     6879      +21     

Impacted Files	Coverage Δ
src/cr2image.cpp	9.72% <0.00%> (ø)
src/crwimage.cpp	71.01% <ø> (ø)
src/http.cpp	0.00% <0.00%> (ø)
src/ini.cpp	6.73% <0.00%> (ø)
src/olympusmn_int.cpp	20.46% <0.00%> (ø)
src/orfimage.cpp	44.59% <0.00%> (ø)
src/rw2image.cpp	43.90% <0.00%> (ø)
src/value.cpp	80.26% <0.00%> (ø)
unitTests/test_cr2header_int.cpp	100.00% <ø> (ø)
unitTests/test_slice.cpp	100.00% <ø> (ø)
... and 38 more

---

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[neheb]

Rebased.

[piponazo]

LGTM, thanks!