mtest: do not process zero byte reads in read_decode()

AsyncIO.StreamReader.readuntil() occasionally raises IncompleteRead
exception before a byte of data has been read. Do not process the "read"
data in those cases.

mesonbuild/mtest.py

diff --git a/mesonbuild/mtest.py b/mesonbuild/mtest.py
index 11e4d31..b5182cc 100644
--- a/mesonbuild/mtest.py
+++ b/mesonbuild/mtest.py
@@ -1123,10 +1123,11 @@
                 line_bytes = e.partial
             except asyncio.LimitOverrunError as e:
                 line_bytes = await reader.readexactly(e.consumed)
-            line = decode(line_bytes)
-            stdo_lines.append(line)
-            if console_mode is ConsoleUser.STDOUT:
-                print(line, end='', flush=True)
+            if line_bytes:
+                line = decode(line_bytes)
+                stdo_lines.append(line)
+                if console_mode is ConsoleUser.STDOUT:
+                    print(line, end='', flush=True)
         return ''.join(stdo_lines)
     except asyncio.CancelledError:
         return ''.join(stdo_lines)