image-info: ignore lines with only whitespace characters in fstab

image-info could produce a weird fstab error with an empty list as a
member, when analysing images not built using osbuild. Ensure that any
lines in fstab with only whitespace characters are skipped during image
analysis.

Signed-off-by: Tomas Hozza <thozza@redhat.com>

tools/image-info

@@ -555,7 +555,6 @@ def read_fstab(tree):
 
     An example return value:
     [
-        [],
         [
             "UUID=6d066eb4-e4c1-4472-91f9-d167097f48d1",
             "/",
@@ -569,7 +568,7 @@ def read_fstab(tree):
     result = []
     with contextlib.suppress(FileNotFoundError):
         with open(f"{tree}/etc/fstab") as f:
-            result = sorted([line.split() for line in f if line and not line.startswith("#")])
+            result = sorted([line.split() for line in f if line.strip() and not line.startswith("#")])
     return result