fix: serialize booleans as lowercase keywords in create_dictionary_string

[agustin18]

Description

In create_dictionary_string() (fpdf/syntax.py), dictionary entries are serialized by stringifying keys and values:

field_join.join(key_value_join.join((k, str(v))) for k, v in dict_.items())

When a dictionary contains native Python boolean values (True or False), calling raw str(v) produces uppercase True or False (e.g. <</Marked True>>), and has_empty_fields=True inadvertently stripped False and 0 due to if v.

This violates the PDF 1.7 / 2.0 specification (ISO 32000-1 § 7.3.2: boolean objects must be written as the lowercase keywords true and false).

This fix:

1. Serializes boolean values with str(v).lower() in create_dictionary_string(), ensuring strict compliance with ISO 32000-1 § 7.3.2.
2. Ensures clear_empty_fields() preserves significant falsy values (False and numeric 0) while filtering out None and empty values.
3. Extends test/test_syntax.py with parameterized test coverage for boolean dictionary serialization.