tools: kconfig2html: Handle blank input lines

[Old-Ding]

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Summary

read_line() removes a trailing newline and then checks g_line[len - 1] for a line-continuation backslash. A blank input line leaves len at zero, so the continuation check would read before g_line.

Return the empty line to the caller before checking for a continuation. kconfig_line() already skips empty lines, so parser behavior for non-empty Kconfig entries stays unchanged.

Impact

No configuration syntax, generated output, or hardware behavior changes. Blank Kconfig lines are now handled safely instead of reaching the continuation-marker access with an empty buffer.

Testing

• Host: Windows 11; compiled tools/kconfig2html.c with GCC 16.1.0 (winlibs).
• Created a minimal Kconfig fixture containing blank lines, then ran the compiled kconfig2html; it exited successfully and generated a 4,177-byte HTML file.
• git diff --check passed.
• A full-tree run was also attempted. It stops at the existing Unrecognized garbage after default value parser limitation, outside the changed read_line() blank-line path; no hardware testing applies to this host tool change.

[github-actions]

MemBrowse Memory Report

arduino-mega2560

• flash: .text +48 B (+0.1%, 67,248 B / 262,144 B, total: 26% used)

esp32-devkitc

• ROM: .flash.text +56 B (+0.0%, 122,708 B / 4,194,272 B, total: 3% used)
• irom0_0_seg: .flash.text +56 B (+0.1%, 87,076 B / 3,342,304 B, total: 3% used)

hifive1-revb

• flash: .text -8 B (-0.0%, 83,244 B / 4,194,304 B, total: 2% used)

mirtoo

• kseg0_progmem: .text +64 B (+0.1%, 67,152 B / 131,072 B, total: 51% used)

qemu-armv8a

• Code: .text.arm64_fpu_procfs_stat -4 B, .text.file_vioctl -4 B, .text.host_open -4 B, .text.inode_stat -4 B, .text.posix_spawnattr_init -16 B, .text.task_spawn +36 B, .text.uart_readv +48 B (+0.0%, 315,362 B)

qemu-intel64

• Code: .text +85 B (+0.0%, 8,655,434 B)

rx65n-rsk2mb

• ROM: .text +48 B (+0.1%, 85,656 B / 2,097,152 B, total: 4% used)

s698pm-dkit

• Code: .text +816 B (+0.2%, 358,096 B)

stm32-nucleo-f103rb

• flash: .text +52 B (+0.2%, 33,788 B / 131,072 B, total: 26% used)

[linguini1]

I think you should try running the tool to make sure that your change does what you want

[linguini1]

Can you also edit the description to follow the PR template?