Empty MISE_GITHUB_TOKEN is not valid

[Methy659]

Describe the bug
If you have a invalid GITHUB_TOKEN set, setting MISE_GITHUB_TOKEN to empty does not skip the rest of the checks. - same issue as: #2910

The changes made in https://github.com/jdx/mise/pull/2915/files#diff-4da8e8ee5c65a27bd048a2f2e55de0a9c75e87618fc02e17b235cc5413e625a0R160 does not respect empty value for MISE_GITHUB_TOKEN.

To Reproduce
#2910 (comment)

# Base image
FROM debian:bullseye-slim

# Install dependencies
RUN apt-get update && apt-get install -y \
    curl \
    bash \
    gnupg \
    && apt-get clean && rm -rf /var/lib/apt/lists/*

# Install mise
RUN curl https://mise.run | MISE_INSTALL_PATH=/usr/local/bin/mise sh && \
    echo 'eval "$(mise activate bash)"' >> /etc/profile && \
    mkdir -p .local/share/mise

# Set environment variables
ENV MISE_GITHUB_TOKEN=""
ENV GITHUB_TOKEN="invalid_token"

# Set cmd to attempt installing a package using mise
CMD bash -c "/usr/local/bin/mise --version && /usr/local/bin/mise install github-cli@2.4.0"

Expected behavior
Treats MISE_GITHUB_TOKEN as a valid value even if it’s empty, stopping further lookups.