python-format: Accept null precision.

Proof that it's valid:
$ python
print("%(value).f" % { 'value': 3.1415926535 })

* gettext-tools/src/format-python.c: Fix comment regarding the precision.
* gettext-tools/tests/format-python-1: Add a test case with null precision.

gettext-tools/src/format-python.c

@@ -50,7 +50,7 @@
    - is optionally followed by a width specification: '*' (reads an argument)
      or a nonempty digit sequence,
    - is optionally followed by '.' and a precision specification: '*' (reads
-     an argument) or a nonempty digit sequence,
+     an argument) or an optional nonempty digit sequence,
    - is optionally followed by a size specifier, one of 'h' 'l' 'L'.
    - is finished by a specifier
        - '%', that needs no argument,

gettext-tools/tests/format-python-1

@@ -64,6 +64,8 @@ cat <<\EOF > f-p-1.data
 "abc%y"
 # Invalid: flags after width
 "abc%*0g"
+# Valid: null precision
+"abc%.f"
 # Invalid: twice precision
 "abc%.4.2g"
 # Invalid: two size specifiers