dpkg-shlibdeps: Handle empty minimum versions when initializing them

If the minimum version is empty (which means it was 0), we cannot use it
to compare against the existing minimum version if that exists, and we
should not be overwritting that, either.

Regression introduced in commit 51c55641fb6c66e149ea93c344cb788c68978c75.

Closes: #764929

debian/changelog

@@ -1,5 +1,10 @@
 dpkg (1.17.18) UNRELEASED; urgency=low
 
+  [ Guillem Jover ]
+  * Handle empty minimum versions when initializing dependency versions,
+    as the code is mapping the minimum version 0 to '' to avoid outputting
+    useless versions. Regression introduced in dpkg 1.17.17. Closes: #764929
+
   [ Updated programs translations ]
   * Catalan (Guillem Jover).
 

scripts/dpkg-shlibdeps.pl

@@ -617,7 +617,7 @@ sub update_dependency_version {
 	if (exists $dependencies{$cur_field}{$subdep} and
 	    defined($dependencies{$cur_field}{$subdep}))
 	{
-	    if ($dependencies{$cur_field}{$subdep} eq '' or
+	    if ($dependencies{$cur_field}{$subdep} eq '' or $minver ne '' and
 		version_compare_relation($minver, REL_GT,
 				         $dependencies{$cur_field}{$subdep}))
 	    {