Update test for empty DataFrame.

libhts/libhts.py

@@ -648,7 +648,8 @@ def plot_MA(res,
     It can also be "lfcMLE", which is based on uncorrected values.
     This may not be good for genes with low expression levels."""
     # if not len(res):
-    if not res:
+    # if not res:
+    if res.empty:
         raise ValueError("No data to plot.")
     fig, axis = plt.subplots()
     # Make a column indicating whether the gene is DE or NS
@@ -746,7 +747,8 @@ def plot_scatter(data,
     # No rows
     # if not len(data):
     # Does it work like that too?
-    if not data:
+    # if not data:
+    if data.empty:
         raise ValueError("No data to plot.")
     # fig, axis = plt.subplots()
     _, axis = plt.subplots()