[SV 28456] Don't override $< when no default rule has been defined

[SV 28456] Don't override $< when no default rule has been defined

The check for matching a file's command to the default rule's command
does not account for null. If no .DEFAULT is defined a rule with no
recipe has it's $< variable set to the value of $@. This breaks second
expansion, particularly when used with pattern rules.

* src/commands.c [set_file_variables]: Check that cmds is set
* tests/scripts/features/se_explicit: Test case
* tests/scripts/features/se_implicit: Test case

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