Fix empty subtask string split

Bug introduced in commit eb32c72.

index.html

@@ -38,7 +38,7 @@ <h1 style=margin:0>HTN Plan Viewer</h1>
       data[m[1]] = ["\n  " + m[1] + ' [label="' + m[2] + '" shape=box3d]', []];
     else if(m = l.match(/^(\d+)\s+([\w\s()-]+?)\s+->\s+([\w()-]+)\s*([\d\s]*)$/)) // Method
       if(m[2] === "__top" && m[3] === "__top_method") order = m[4];
-      else data[m[1]] = ["\n  " + m[1] + ' [label="' + m[2] + '" shape=rectangle]; m' + m[1] + ' [label="' + m[3] + '"]; ' + m[1] + " -> m" + m[1] + " -> {" + m[4] + "}", m[4].split(/\s+/)];
+      else data[m[1]] = ["\n  " + m[1] + ' [label="' + m[2] + '" shape=rectangle]; m' + m[1] + ' [label="' + m[3] + '"]; ' + m[1] + " -> m" + m[1] + " -> {" + m[4] + "}", m[4] ? m[4].split(/\s+/) : []];
     else if(m = l.match(/^root\s+([\d\s]*)$/)) // Root
       order = m[1];
     else warnings += "Ignoring line " + (i+1) + ": " + l + "\n";