ZJIT: Handle `nil` case for `getblockparamproxy` (#15986)

Resolves https://github.com/Shopify/ruby/issues/772

Adds profiling for the `getblockparamproxy` YARV instruction and handles the `nil` block case by pushing `nil` instead of the block proxy object, improves `ratio_in_zjit` a tiny bit (0.1%)

Profiling data for `getblockparamproxy` on Lobsters
```
Top-6 getblockparamproxy handler (100.0% of total 3,353,291):
  polymorphic: 2,337,372 (69.7%)
          nil:   552,629 (16.5%)
         iseq:   259,636 ( 7.7%)
  no_profiles:   156,734 ( 4.7%)
         proc:    40,223 ( 1.2%)
  megamorphic:     6,697 ( 0.2%)
```

Lobsters benchmark stats:
<details>
<summary>Stats before (master):</summary>
<p>

```
❯ ./run_benchmarks.rb --chruby 'ruby-zjit --zjit-stats' lobsters
***ZJIT: Printing ZJIT statistics on exit***
...
Top-20 side exit reasons (100.0% of total 15,338,024):
                   guard_type_failure: 6,889,050 (44.9%)
                  guard_shape_failure: 6,848,898 (44.7%)
  block_param_proxy_not_iseq_or_ifunc: 1,008,525 ( 6.6%)
                   unhandled_hir_insn:   236,977 ( 1.5%)
                        compile_error:   191,763 ( 1.3%)
                 fixnum_mult_overflow:    50,739 ( 0.3%)
           block_param_proxy_modified:    28,119 ( 0.2%)
     patchpoint_stable_constant_names:    18,229 ( 0.1%)
         unhandled_newarray_send_pack:    14,481 ( 0.1%)
                  unhandled_block_arg:    13,782 ( 0.1%)
               fixnum_lshift_overflow:    10,085 ( 0.1%)
              patchpoint_no_ep_escape:     7,815 ( 0.1%)
                  unhandled_yarv_insn:     7,540 ( 0.0%)
                  expandarray_failure:     4,533 ( 0.0%)
             guard_super_method_entry:     4,475 ( 0.0%)
          patchpoint_method_redefined:     1,207 ( 0.0%)
        patchpoint_no_singleton_class:     1,130 ( 0.0%)
               obj_to_string_fallback:       412 ( 0.0%)
                   guard_less_failure:       163 ( 0.0%)
                            interrupt:        82 ( 0.0%)
...
ratio_in_zjit:                                     82.1%
```
</p>
</details>

<details>
<summary>Stats after:</summary>
<p>

```
❯ ./run_benchmarks.rb --chruby 'ruby-zjit --zjit-stats' lobsters
***ZJIT: Printing ZJIT statistics on exit***
...
Top-20 side exit reasons (100.0% of total 15,061,422):
                   guard_type_failure: 6,892,934 (45.8%)
                  guard_shape_failure: 6,850,512 (45.5%)
  block_param_proxy_not_iseq_or_ifunc:   549,823 ( 3.7%)
                   unhandled_hir_insn:   236,979 ( 1.6%)
                        compile_error:   191,782 ( 1.3%)
                  unhandled_yarv_insn:   128,695 ( 0.9%)
            block_param_proxy_not_nil:    68,623 ( 0.5%)
                 fixnum_mult_overflow:    50,739 ( 0.3%)
     patchpoint_stable_constant_names:    18,568 ( 0.1%)
         unhandled_newarray_send_pack:    14,481 ( 0.1%)
           block_param_proxy_modified:    13,819 ( 0.1%)
                  unhandled_block_arg:    13,798 ( 0.1%)
               fixnum_lshift_overflow:    10,085 ( 0.1%)
              patchpoint_no_ep_escape:     7,815 ( 0.1%)
                  expandarray_failure:     4,533 ( 0.0%)
             guard_super_method_entry:     4,475 ( 0.0%)
          patchpoint_method_redefined:     1,207 ( 0.0%)
               obj_to_string_fallback:     1,140 ( 0.0%)
        patchpoint_no_singleton_class:     1,130 ( 0.0%)
                   guard_less_failure:       163 ( 0.0%)
...
ratio_in_zjit:                                    82.2%
```
</p>
</details>

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