Fix TypeError in Grid#apply_gutter when gutter is not specified

[55728]

Closes #1343
Closes #1344

Problem

Since commit be66ddd ("Update code style"), define_grid raises a TypeError when called without gutter options:

define_grid(columns: 5, rows: 8)
# => TypeError: can't convert nil into Float

The same error occurs when only one of row_gutter or column_gutter is specified:

define_grid(columns: 5, rows: 8, row_gutter: 10)
# => TypeError: can't convert nil into Float (from column_gutter)

The cause is that apply_gutter was changed from options[:row_gutter].to_f (which returns 0.0 for nil) to Float(options[:row_gutter]) (which raises TypeError for nil).

Solution

Use Hash#fetch with a default value of 0:

@row_gutter = Float(options.fetch(:row_gutter, 0))
@column_gutter = Float(options.fetch(:column_gutter, 0))

This preserves the stricter Float() conversion for invalid values while defaulting to zero when options are omitted.

Tests

Added three test cases covering:

• No gutter options specified at all
• Only row_gutter specified (column_gutter defaults to 0)
• Only column_gutter specified (row_gutter defaults to 0)

Note

@afdev82 previously addressed this in their fork (adnotam/prawn@327de7e) but a PR was not opened against upstream. This PR follows a similar approach.