Fix make empty variable warnings #11332

Job	Run time
cppcheck	2m 10s
package-indep (debian:bullseye)	43m 43s
htmldocs	44m 18s
package-indep (debian:bookworm)	34m 39s
package-arch (debian:bullseye)	28m 56s
rip-and-test	28m 8s
package-indep (debian:sid)	39m 33s
rip-and-test-clang	27m 29s
package-arch (debian:bookworm)	29m 47s
package-arch (debian:sid)	31m 34s
	5h 10m 17s

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