YJIT: Speculate block arg for c_func_method(&nil) calls

[XrXr]

A good amount of call sites always pass nil as block argument, but the
nil doesn't show up in the context. Put a runtime guard for those
cases to handle it. Particular relevant for the ruby-lsp benchmark in
yjit-bench. Up to a 2% speedup across headline benchmarks.

master: ruby 3.4.0dev (2024-12-12T17:26:06Z master c0caf1cc1a) +YJIT +PRISM [x86_64-linux]
+patch: ruby 3.4.0dev (2024-12-12T18:42:41Z +patch c639b0918b) +YJIT +PRISM [x86_64-linux]

--------------  -----------  ----------  -----------  ----------  --------------  -------------
bench           master (ms)  stddev (%)  +patch (ms)  stddev (%)  +patch 1st itr  master/+patch
activerecord    186.6        0.7         185.6        0.7         1.001           1.005        
chunky-png      503.3        0.1         501.8        0.1         1.000           1.003        
erubi-rails     924.8        0.6         922.9        0.6         0.980           1.002        
hexapdf         1903.8       2.5         1986.2       2.8         1.015           0.958        
liquid-c        56.3         1.2         55.7         1.3         1.004           1.012        
liquid-compile  57.9         1.9         58.1         1.9         0.988           0.998        
liquid-render   81.4         1.1         80.0         1.3         1.009           1.018        
lobsters        862.3        1.5         864.1        1.6         1.048           0.998        
mail            120.8        0.6         120.8        0.7         1.001           1.000        
psych-load      1638.7       0.1         1622.9       0.0         1.010           1.010        
railsbench      1944.8       0.3         1941.2       0.1         1.001           1.002        
rubocop         112.6        6.7         112.2        6.6         0.997           1.003        
ruby-lsp        131.8        2.0         120.5        2.0         1.024           1.094        
sequel          68.8         0.7         67.0         1.0         1.032           1.027        
--------------  -----------  ----------  -----------  ----------  --------------  -------------
Legend:
- +patch 1st itr: ratio of master/+patch time for the first benchmarking iteration.
- master/+patch: ratio of master/+patch time. Higher is better for +patch. Above 1 represents a speedup.

hexapdf seems to have a lot of run-to-run variation. On a repeat run:

master: ruby 3.4.0dev (2024-12-12T17:26:06Z master c0caf1cc1a) +YJIT +PRISM [x86_64-linux]
+patch: ruby 3.4.0dev (2024-12-12T18:42:41Z +patch c639b0918b) +YJIT +PRISM [x86_64-linux]

-------  -----------  ----------  -----------  ----------  --------------  -------------
bench    master (ms)  stddev (%)  +patch (ms)  stddev (%)  +patch 1st itr  master/+patch
hexapdf  1996.2       2.7         2012.2       2.2         1.011           0.992        
-------  -----------  ----------  -----------  ----------  --------------  -------------
Legend:
- +patch 1st itr: ratio of master/+patch time for the first benchmarking iteration.
- master/+patch: ratio of master/+patch time. Higher is better for +patch. Above 1 represents a speedup.

So I want to say this patch doesn't cause any headliners to slowdown.