[PkgConfig] Avoid creating blank dependency name

This can be seen in some awkwardly generated .pc files and
it can crash SwiftPM silently at least on Windows.

Author: stevapple

Sources/PackageLoading/PkgConfig.swift

@@ -271,7 +271,7 @@ internal struct PkgConfigParser {
                         \(pcFile)
                         """)
                 }
-            } else {
+            } else if !arg.isEmpty {
                 // Otherwise it is a dependency.
                 deps.append(arg)
             }