Fix undefined behavior in Y4M header parsing
The OpenY4mFile function parses the video header by tokenizing the string with spaces.
If a header contains consecutive spaces (e.g., "W100  H100"), the tokenizer produces empty strings.

Currently, the code calls `field.front()` on these empty strings. Accessing `front()` on an empty container is Undefined Behavior in C++ and can cause crashes on strict platforms.

This CL adds a check to explicitly skip empty fields, making the parser robust against extra whitespace.

Bug: None
Test: Verified locally by parsing a Y4M header with double spaces.
Change-Id: Icb477d722fb496a055daecdf1f1007cfd43fe9d0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/447000
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46846}