If 'exports' statement uses package name, then this package should contain file with package declaration due to assertion jigsaw-1.1.2-210:

If the named package is observable in the current module, then at least one compilation unit containing a declaration of the package must be observable in the current module, or a compile-time error occurs.

So compile-time error is expected if the only file is empty (has only comments). But no error occurs.

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Minimal case:

Directory layout:
mod1/module-info.java
mod1/pack1/A.java

Code:
A.java:
{code}
// No package declaration here.
{/code}