pmb.aportgen.core.rewrite: no lines with empty val (MR 1942)

Avoid apkbuild-lint errors like:
	[AL5]:APKBUILD:15:variable set to empty string

pmb/aportgen/core.py

@@ -118,7 +118,11 @@ def rewrite(args, pkgname, path_original="", fields={}, replace_pkgname=None,
             # Replace fields
             for key, value in fields.items():
                 if line.startswith(key + "="):
-                    line = key + "=\"" + value + "\"\n"
+                    if value:
+                        line = key + "=\"" + value + "\"\n"
+                    else:
+                        # Remove line without value to avoid lint error
+                        line = ""
                     break
 
             # Replace $pkgname

test/testdata/aportgen/pmaports/cross/binutils-armhf/APKBUILD

@@ -8,12 +8,9 @@ pkgrel=1
 pkgdesc="Tools necessary to build programs for armhf targets"
 url="https://www.gnu.org/software/binutils/"
 depends=""
-makedepends_build=""
-makedepends_host=""
 makedepends="gettext libtool autoconf automake bison texinfo"
 arch="armv7 aarch64 x86_64 x86"
 license="GPL-2.0 GPL-3.0-or-later LGPL-2.0 BSD"
-subpackages=""
 source="https://ftp.gnu.org/gnu/binutils/binutils-$pkgver.tar.bz2
 	x86-Add-a-GNU_PROPERTY_X86_ISA_1_USED-note-if-needed.patch
 	x86-Properly-merge-GNU_PROPERTY_X86_ISA_1_USED.patch