Ignore empty "includes" field in library.properties

This allows to avoid empty include lines like:

   #include <>

for libraries declaring:

   includes=

in their library.properties file.

Fix #7854

Author: cmaglie

arduino-core/src/processing/app/packages/UserLibrary.java

@@ -141,7 +141,7 @@ public static UserLibrary create(UserLibraryFolder libFolderDesc) throws IOExcep
     }
 
     List<String> includes = null;
-    if (properties.containsKey("includes")) {
+    if (properties.containsKey("includes") && !properties.get("includes").trim().isEmpty()) {
       includes = new ArrayList<>();
       for (String i : properties.get("includes").split(","))
         includes.add(i.trim());