Fix indent filter ignoring blank=False for empty first line

[sapirbaruch]

Fixes #2176.

When first=True is passed to the indent filter, the code was
unconditionally prepending the indentation string to the output, without
checking whether blank=False should suppress it when the first line is
empty.

Before:

>>> env.from_string('{% filter indent(4, first=true) %}{% endfilter %}').render()
'    '  # four spaces added to an empty string

After:

>>> env.from_string('{% filter indent(4, first=true) %}{% endfilter %}').render()
''  # blank=False (default) suppresses indentation of the empty first line

The fix is a one-liner in do_indent: instead of if first:, check
if first and (blank or rv.split("\n", 1)[0]): so that an empty first
line is only indented when blank=True is explicitly requested.

A new test (test_indent_first_blank) covers the three cases:

1. Empty string with first=True → no indentation
2. Leading blank line with first=True → blank line left alone, rest indented
3. Empty string with first=True, blank=True → indentation applied

[sapirbaruch]

The PyPy failure is a pre-existing trio incompatibility that affects other open PRs too — not caused by this change. All other checks pass.