Fix IndentationError when a subschema generates no validation code

[gaoflow]

compile() generates invalid Python for some valid schemas:

>>> fastjsonschema.compile({"not": {"title": "x"}})
IndentationError: expected an indented block after 'try' statement on line 4

Any truthy subschema that produces no validation code triggers this: annotation-only schemas (title, description, $comment, ...), unknown keywords, or no-op keywords such as {"not": false}, {"uniqueItems": false}, or an unrecognized format (a no-op since 63e29e3). The generator emits a block header with nothing under it.

This is the same class as #204/#205, so I probed every subschema position of every generator with code-less subschemas across drafts 04/06/07/2019. Seven sites are affected:

• not (try body) — unguarded
• then and else of if-then-else — 263ecad guarded only the if block
• schema-form dependencies — unguarded
• single-schema items, additionalItems, propertyNames — guarded, but by the number of keyword functions run (the return value of generate_func_code_block), which misses keywords that run and emit nothing: {"items": {"not": false}} still crashed

The fix applies the code_len snapshot pattern from 263ecad at all seven sites, replacing the count-based guards. The remaining generators (anyOf/oneOf/contains/properties/patternProperties/additionalProperties) always emit at least one body line and were verified unaffected.

This also fixes the compile crash on the suite's optional/unknownKeyword.json — three of its cases now pass, but the file has to stay in ignored_suite_files because its other cases need $ref-to-embedded-$id resolution, which is a separate gap.

Independent of #181, which fixes a different generate_not bug (not: {} accepting falsy instances); the two compose.

[horejsek]

Thank you!